Heres the way I look at it.
If you want something that will scare the hell out of you, be fast stock, faster with VERY little money in it, but have to deal with little nicknack problems all the time (window motors), go with the LS1.
If you want something slower (lets face it, now-a-days our cars arent all that fast), but TONS of fun, fewer BS problems all the time, and tons of value for the dollar, go with the Mustang. Id LOVE to have a dark blue SS sitting on chrome Z06 wheels, but at the same time, my Mustang is SO much fun and almost paid for, so why switch now.