The M3 is going to make the FRS feel very slow (power wise). I track a non-M e36 (but with a lot of M upgrades) as well as my BRZ, and the e36 accelerates noticeable quicker than my BRZ. The BRZ is overall a faster car on the track though. Consumables are pretty close in cost and not bad at all. The e36's can also be pretty reliable if you do the "normal" maintenance on them.
It's a tough choice but I don't think you can go wrong either way.
BTW, where is this M3 because if you don't buy I might to swap my good non-M parts into
