Stock, the V6 looks like it gets 6 more MPG than the V8 under similar driving conditions. Having the V6, I think the only reason to get a V6 is how many miles you put on it a week. I put 500+ miles a week. This is definitely a daily driver for me.
Using regular gas ($3/gal in California), this works out to be about $12 a week or about $624 a year.
It depends how much that $520 means to you. It wasn't just the gas that's a big deal to me. It was the fact with all the miles I put on the car, it's going to last me 4 years.
And that assumes I don't go on road trips. I don't keep any car past 100k.
Your insurance will be slightly less with the V6. Not by much especially if you are older, only maybe a hundred or two. People have reported that the V6 Nissan 370Z costs more than a V8 Mustang.
I work in a dot-com, so I can afford a better Stang. If I was to get a job closer to home, I would trade this in a heartbeat. But as it is now, it's far better deal for me to drive a base model auto V6 and trick out the mileage.
To get the 34 MPG, I added lighter wheels: 255/45/18 (22lbs rims I think), K&N, and Bama Tune. I really only get that when a cop is around and I am forced to drive slow.