To me, it was mainly the price. I cannot find a reason on why I should paid nearly 50K out the door to get a 330i (in So Cal).
I have to say I've never driven the 330i, but from my experience of the 323i/ci, 328i/ci, and 540 sports (auto), I have to say that BMW is not really the Ultimate Driving Machine I thought it would be. I have to say the handling is pretty good, although I don't think it is top notch, and may lack the IS a little bit. Also, the steptronic sucks. It takes forever to upshift, and this happen on all the 3 series I test drove and my friend's new 540 sport. When shifting at the redline, you will have to take your foot out of the gas padel to prevent it going too far into the redline. I don't think BMW will improve the steptronic that much, if at all, on the 330i. If you really want one, get a stick.
This is one thing I really like about the 3 series. It's transmission is smooth, probably best I've ever drove. The clutch is easy to operate (unlike my old eclipse) and light, while the gear selector is gated really well and you probably won't miss a gear. And if you like to try heel-and-toe, the pedals are setup corretly for you to try this.
Oh another thing, watch for the reliability of the car. I think the car will be great to have for the first 36,000miles, but after that, prepare to pull some big bucks from your wallet when going in for a service.
One thing I do have to give to the BMW is its comfort. It rides a little bit smoother than the IS, so if that is what you are looking for, go with BMW.