E46 M3 for a few reasons:
1. HPFP (High Pressure Fuel Pump). The 135 and 335 share this part, and the failure rate (and re-fail rate) is very high. The E46 M3 engine doesn't suffer this problem. (Simply search on this issue on the E90 or 1Addicts boards...it's huge). I'd rather enjoy my car than deal with this.
2. The N/A high-Rev enjoyment of the E46.
3. Heat soak--if you want to track the car even with the oil cooler the N54 suffers from heat soak. It's kinda warm in Oz (haven't been, but want to) from what I hear, so this should be a consideration. The N54 just runs hot to begin with.
3. Handling, precision.
4. Looks. The E46 still looks awesome. I only saw mainly the back end of this one
most of the day yesterday on some "spirited" runs on some back roads....and it looked pretty damn good while staying in front of me!