Buy the Cayman, then have your 800-pound girlfriend sit in the passenger seat. You will then have the equivalent of an M3.

 

You are comparing two very different cars!

M3 = you could have a 2-door coupe or 4-door version to compare against and both with much more luggage and seating space = weight and different driving dynamics.

Cayman = 2-door coupe that is mid-engined and that in itself means you need to go drive both.

Pop down to Al Naboodah Porsche and seek a test drive or go to AGMC BMW and see if you can test drive the M3. Then come ask us all questions if you still have some.

That's my $0.02