I hear ya, if making power is your thing I'd go for the stang. but do drive the m3 first so you won't have any regrets. the m3 is definitely not cheap to mod relative to the 5.0, but to many people it's worth the price tag; is it to you?
I still stick around this board because M cars are a special breed - it doesn't need all the power in the world to offer a sublime driving experience. my z4m is no less fun to drive than my vette even though it's not as quick in stock form or mod for mod.