The lines of the '12 and '13 are the same. The front and rear fascias are different. The interiors are different. On the '13, those things are better. Significantly. Honestly, if the '13 had a keyless fob with push-button-start and I-don't-ever-have-to-take-it-out-of-my-pocket functionality (kinda like, oh, EVERY OTHER auto maker at this point) rather than the same stupid ******* key they've been using for the past decade, I would make a real effort to get rid of my car, get a '13 Si and do all the stuff I had originally planned for my '12. The '13 Si sedan is one of the best looking cars out there in my personal opinion. Keyless fob, leather, four/two piston caliper brake set up, and 60-70 more horsepower and it would've been the absolute best ******* car in its class hands down. The '12....well, it'd need to be a '13 with all that other stuff. The more I think about it, the more I regret buying it and the more pissed off I am to be stuck with it. Not that it's a bad car, but for the price I could have had something better. If I went used, I could have had something MUCH better.
All that said, when I'm driving it and not paying any attention to anything else but the driving experience; the steering, the shifter, the pedal feel, the seat, etc. I absolutely love it. It's only when I'm looking at the (comparatively) cheap looking dash, the front end (mostly that awful grill), and when I have to start and stop the engine with a key in the steering column like I'm driving a car from the previous century, that I am disappointed. That and the fact I've had to dump an extra $6k into it to make it a little better and it still needs at least another $4k to be fast enough to tolerate. For $34k ($24k + $10k to make it fun enough for me not to be completely bored with it after a month) I could have gotten a '11 335i with every option package available.
Alright, I'm done complaining...for now.