Devil's advocate here with a couple things to think about. I don't know if they're material, but they're worth considering:
- Insurance on an "old" 335 vs a new M3?
- Do you care about gas mileage? It will cut pretty significantly
- Care and feeding, though you'll have a new car, so it shouldn't be an issue. If you track the car, brakes and tires will cost more (though it will be more fun).
- 4 doors vs 2. This is a logistical issue if you have family, drive friends around, etc. I also like opening the rear doors of my car and throwing in my track tires for schools (Try THAT with a coupe). You can take it for granted, but I've come to appreciate 4 drs in my 17 yrs of driving and tracking cars...all aesthetics aside. If you still want an M and 4 drs, you can backtrack and buy a slightly used sedan.
- Some owners say (including people with BOTH cars in their garage) that for the realistic proposition of bumping around town in a daily driver, the low and grunt of the 335 is better than the high end howl of the M3. I can barely get my 250HP E36 up the rev band, though I live in a very crowded city/region.
- If you track the car, then the M3 makes infinite sense. If you're realistically not using the car at that level (Don't even have some open country roads to play on, then a 335 might be a more pragmatic choice.
How about you keep the 335 for a year and see how the F80 pans out? You'd get to pick up a slightly used E92 for a steep discount or a new F80 if you're still interested in an //M. There's also the matter of how attractive your buyout on the 335 is. You didn't mention that.