I thought about the 6 for the very same reason, and then went with the 8 for a couple reasons. One, like you said, what is the point of a muscle car without a V8. Two, I sort of came to the conclusion that the 4.0L V6 in the Mustang is intentionally bad. I think it is a really old design, and perhaps even then it is engineered to perform poorly. Ford does not want it to step too much on the toes of the V8. Ford does have a very well engineered 3.5L V6 that produces much more horses (270-ish) than the 4L V6 they put in the Mustang. Were they to put it in the Mustang, the V8 would seem like less of bargain. Plus, the V8 was engineered as a work horse engine. They put it in trucks, and more importantly, they put it in those Ford LTDs cabbies and police use, so the engine was designed to get 200k+ miles. It is a very good engine, compared to the V6, which seems like an afterthought. Finally.... DarkFire said it best, put a good CAI and tune, you can get better mileage than the V6. (of course, I am not clear what kind of mileage you can get with a good intake and tune on the sixer)
Long story short, I went with the V8, and have no regrets. If your thinking about a V8, you will regret not having one.
Oh and... gas prices are falling. It looks like, (knock on wood), that the high gas prices were caused by a speculation bubble. Now, supply outstrips demand, and prices are falling. Lets hope they hit the floor, as we see more electric cars and hybirds, and ethanol production.