The main reason why I got into the M is because of it's a high revving engine - which is what I missed most about my AP1 S2000 (totaled it - not my fault
The lightness is on a whole 'nother level than the 350Z (to my it's a pig) I believe they're 2900lbs. The only downside (if you call it that) was the loudness. But I absolutely loved to VTEC it in a tunnel. And who can argue with 120HP per liter??
If you're thinking of a RELIABLE, cost-effective track car... S2000 hands down.