I say E92 335is.
650i isn't a sports car, it's a GT. 650i will NOT beat a stock 335i. 335i does the 1/4 in 13.2, 650i does it in 13.4. Times are really too close, but 335i will be much faster in the real world because it weighs 300lbs less and has a torque peak 2000rpm sooner. This is before you buy any kinds of mods...
Just depends what you want, OP. If you're under 40, I'd go E9X.
2009 E90 328i - Space Grey Metallic on Black // iDrive // Premium Package // Xenons
Mods - Stock for now
Dream Car - Mineral White E92 M3 DCT, Ferrari 458 (someday )