True, gas will be better.
Insurance? The difference depends on the person, mine would only be about $20 a month more (which I pay $120 anyway so to me its not that much different).
If I could choose between a supercharged v6 with 340hp or a stock GT with 300hp, i'd choose the v8 in a heartbeat. I guess i'd rather have a GT with all that potential (mod-wise) than a v6 that can beat stock GTs...
And this is coming from a guy who's typically seen around my group of friends, who all have v8 mustangs, as the 'v6 enthusiast.'
Of course it all comes down to the cash you've got in your pocket and/or how much of it you're willing/able to spend. When I bought my car I couldn't afford to buy a GT, as with many people. It's also my belief that the v6 model is plenty fast enough for a 22 year old kid (me!)
Geez, us v6 lovers are dropping like flies huh?

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Black 2005 Mustang v6, 5 speed, IUP, EUP and Leather.
Razzi 1/4 window scoops, Mustang GT Sway Bars, Flowmaster American Thunder Single Exhaust, Clear Front Turn Signal Lenses, Demolet CAI, X-cal2 91 tune, Hurst Comp/Plus Shifter, FRPP T-Lock, Ford 3.73 gears