I have had a similar decision process when I was still working (thanks COVID-19).
I have a 2012 GT/CS and I was offered to test drive a 2016 V6 and GT PP brand new with less than 100 miles on it when i went to look at my 2012.
I test drove the v6, and did NOT like it, but i'm a car guy... so having that v8 sound behind me is almost essential, especially when i spent the past year~ish driving around a 2015 kia optima that I bought new in 2015.
When i test drove the 16 GT, for one... it was an auto, so not really as fun, but it was a PP and I wanted to mostly give those brakes a shot.
My first impression was 'crap, there's no clutch or shifter', but the brakes were great. However... another reason I wanted to test drive the GT is to see how I felt in a GT w/ that s550 interior that looked wildly different in my opinion. My resulting opinion was that It felt more comfortable than the v6, mostly because of the sound, and the leather seats, but something felt off, for one.. yes it was an auto, so that was a negative side for me personally. but something about the car, and the look it had on the outside didn't really give me that 'agressive' feeling. I didn't care for the s550 platform when it was shown off at first, and seeing it in person was only confirming the small details that I didn't like.
Side note: I didn't like the 13-14 models because of the adopted front end that came from the shelby made the years prior, that was that 'hunch' in the front above the front grill. I didn't like that at all.
So, obviously from my first sentence here, I drove the 2012 GT/CS and I don't know why, or what that car was saying to me but it was something similar to 'buy me'. The hood was the style I liked, it draped over the grill and the lights when down, and there was no 'hunch' on the front end, so it immediately looked 1000x better than that 2016 GT PP, It missed the brembos... but I wasn't as impressed w/ the brembo's because the 2012 stopped good enough that I forgot it didn't have brembos.
So, those were those 3 specific models.
First off, the only model year i drove 13-14 were a 13' V6 and V8 (GT). I wasn't interested in the body style of them, I simply wanted to test drive them and see if I felt any difference at all between my car and them.
I didn't feel enough of a difference to warrant trading in my GT/CS on a base model GT... and def. not a v6.
I wasn't really looking for them anyway, I was looking for a second 'daily' after my GT reached about 85k miles.
I wanted a v6, or a cheap GT that had nothing special about it, mileage wasn't a concern, and my insurance company at the time lopped the v6 with the GT and didn't care which I had because I actually AM a safe driver (mostly).
So after that, I went to another dealer and test drove an ecoboost, V6, and a GT all in one go!
I wasn't wasting their time because I showed them I had 2000$ to put down and depending on the car I chose, I had more saved up (then anyway).
So the V6 was first, I wanted to get a pure comparison of the engine feel, driving feel etc.
the v6 was a 16 model, but I wanted to drive it again to see if my mood changed after owning the 2012 GT/CS for so long.
It didn't, i hated the v6, and it was also uncomfortable compared even to my 2012 GT/CS w/ the leather seats and the inserts... and the v6 didn't have heated seats xD.
I passed on it quickly....
The ecoboost was my next choice, and it was a 17 model, less miles, but surprisingly cheaper than I thought, They wanted I think 16.5 for the car on sticker. I thought it might be worth having a 4cyl mustang... if nothing else for the fact it was another mustang and I would still be driving a mustang, even it it wasn't a v8.
The ecoboost didn't disappoint surprisingly, maybe it was because it was newer, or because it had leather seats, (I actually didn't really want leather, but i'm not picky), or maybe because I hadn't felt the 4cyl yet and it surprised me compared to the kia optima I used to have... either way, I almost took that car home, it just felt like a good mid-car between the v8 and a daily. It didn't feel as fast, but it was comfortable, so I left it open as an option.
Then I drove a practically brand new 17 GT, with a PP, because the dealer said the others all had PP and it didn't matter anyway.(they only had like 5 GT's).
So I test drove that GT and I have to admit, I liked the exhaust and exhaust modes, and there was a little more styling done to the car (or i thought there was anyway). The handling felt about the same as my 2012 GT/CS, so nothing impressive, and then the stopping power, obviously after years of using standard brakes, I felt like using brembo's at that point had a little more stopping power, but nothing really that impressive.
The V8 sound instantly relaxed me much more than driving the other 2 cars though... which made me almost want to buy another GT, but in the end, I chose to keep my 2012 GT/CS and wait a year or two to get another daily.
Fast forward and I got laid off (due to a company move across the country before COVID-19).
I worked at an auto-auction nearby, and was able to drive 18+ ecoboost and v8 options, one which was fully loaded.
There are a few small touches that made me like the 18+ bodystyle, such as the hood draping over the lights more, and the front in general seeming more defined than the older ones. Either way, the 4cyl 18 model... felt lighter, I was never allowed to go over 30mph, but around a corner, it felt as if it had some sort of suspension upgrade that made the car feel like it was floating, and the engine exhaust was hardly in my ear, so it wasn't annoying... but in a way it was, being used to a v8 sound.
I felt the same improvements in the 18 GT's, they felt so light, of course the sound was there, which i liked personally, so it was both comfortable and several small touches on the cars made them feel better overall... but in all honesty, I kind of like the rough feel of older cars more.
The 18 manuals also have clutches that feel buttery smooth... which I personally didn't care for, they were so easy to press, and you had to go so far down to the floor for it to engage properly, that my foot actually slipped off the clutch pedal while I was going to shift it into second gear. I blame the detail guys for that though, wiping off everything....
When I went to get a cheap daily i found a 15 ecoboost for 12,500, it had over 50k miles, so ... not the best, but it had a large display, and it was a manual. I was almost sold immediately, come to find out there had been a ding on the rear passenger, they couldn't finance it at all, even with 2k down (not a lot but i couldn't afford more and still pay my GT off
)
So that car got sold shortly after and I never saw it again, but I have a dealer near me looking for a 15-17 ecoboost for me as a daily.
I don't care about any options, except I specified NO leather seats... except I want seat warmers , so that conversation was interesting.
Anyway...
To conclude that massive story more or less... I've driven them all, and my experience will vary from yours greatly, as you may not be as much of a performance enthusiast person as me. I personally think the car needs the v8 to be relaxing.... but i digress.
If you want comfort, you probably want the 15-17 model year, you're already in used car territory, so mileage will literally vary on the cars, so do yourself a favor and self-inspect the cars inside and outside before you make your decision.
If you want the v6, I would honestly go with a 13-14 since it will be cheaper, and it's only usage is as a daily anyway.
Now, if you want a 'real' mustang, then go with a v8, 13-14 or 15-17 and you can't go wrong, you can still get 20MPG on highway (around 80mph if that's your thing) and more if you are 'conservative' and only go 70mph.
However, if fuel economy is your biggest issue, and you travel a long way, go ahead and grab an ecoboost, but I recommend grabbing the 16-17 model year, as the 15 i've had plenty of people tell me that they have varying fuel economy for some reason. I can't back that up, but it's really up to you!
If you want a personal opinion on what I would choose (forgoing the distaste of the body styles) as a daily, I would grab the ecoboost...for the mpg, and just to have a newer body... but that's because I already have a 2012 v8
I would probably mod the 4cyl, 6cyl, or even another v8 if I got it anyway, so for me, my choice is probably negligent xD
I hope my long and fun little story about my experience with all of them helped.
quick side note... I have a friend that owned a v6 2014, and swapped up? to a 16 ecoboost and he says he loves it so much more than the older car and he's only owned it a few months now, so another opinion for you there.