I have a 350z that I track and DD occasionally. All cars will have heat problems when pushed hard on the track but my personal experience is that the M3 is more prone to heat issues overall. I wouldn't describe the Z as having a truck like motor I'd say its more of a muscle car feel, and IMHO the tranny feels better than the M3 one.
Like I said before if I'm buying a car specifically for the track it'd be something small and light like a miata. Adding power is easy but you can't shrink a cars proportions and you can only remove so much weight