Depending on your level of Autocross involvement - fun/local meets vs. serious/regionals & nationals - it could influence your decision. From what I have seen, the 3 Series is very competitive in D Class, the Accura Integra R puts up a good competition. The extra HP on the 330 is nice but not necessarily needed in local events. The M3 in A Stock is competitive,
BUT the Honda S2000, Mitsi EVO, and Subaru WRX STi are also very competitive. We love our bimmers, but having an M3 will not guarantee a win against these guys in A Stock.
So, if you are serious about autocross, you should look at results from local events and consider what class you will be in and what your competition is. That might guide your decision.
M3s run in A Stock Class against these cars:
AS Acura NSX
AS Audi S4 V8 ('04+)
AS
BMW M Coupe and Roadster ('01+) M3 (E46) M5 ('00+)
AS Cadillac CTS-V
AS Chevrolet Camaro SS ('96) Corvette C4 ('84-'96) Corvette ZR-1
AS DeTomaso Mangusta Pantera
AS Ford Mustang Cobra SVT ('03+)
AS Honda S2000 ('04+)
AS Jaguar XKR Coupe
AS Mercedes E55 AMG SLK32 AMG ('02+)
AS Mitsubishi Evolution RS Evolution VIII ('03+)
AS Pontiac Firebird WS6 ('96+)
AS Porsche 911 (993 chassis) non-turbo ('95-'98) Boxster S
AS Saleen Mustang (normally aspirated)
AS Shelby Cobra (all)
AS Subaru WRX Sti
AS Toyota Supra Turbo ('93 1/2+)
3 Series E46 (except the new 330 ZHP) run in D Stock class against these cars:
DS Acura Integra Type R RSX-S ('02+)
DS Audi S4 ('00-'03) (A4 chassis) TT (180hp, single intercooler)
DS
BMW 3 series 6 cyl. (non-M) NOC 330Ci
DS Cadillac CTS
DS Chrysler/Dodge Crossfire Daytona IROC R/T Neon SRT-4
DS Honda Prelude VTEC engine models ('97+)
DS Infiniti G35 Sedan
DS Jaguar X-Type (All NOC) X-Type 3.0 (AWD)
DS Lexus IS300
DS Mazdaspeed Protégé
DS Mercedes C320
DS Mitsubishi/DSM Eclipse/Talon Turbo AWD
DS Nissan Maxima ('04+) Sentra Spec-V ('02+)
DS Saturn Ion Redline
DS Subaru Forester 2.5XT WRX
DS Toyota Celica GTS ('00+)
DS Volkswagen Golf R32 Golf/Jetta/GLI 24V VR6 ('02+)
DS Volvo S60R ('03+) V70R ('03+)
Go here for more class info:
http://www.moutons.org/sccasolo/Lists/2004/stockc.html