Sorry, I thought you know the exact differences between the 2 cars and still want to know if the STi is worth it. If you want facts, metoo basically sums it all up.

Your answer is, YES, the STi is worth every penny and then some. Since you have a bimmer, and I am sure you know all the differences between the E46 330i and the E46 M3. Honestly you can ALWAYS mod ANYTHING over ANYTHING, when it comes to cars, ANYTHING is possible, just depends on how much it cost. The comparison between the 330i/M3 and the WRX/WRX STi is very similar. Like the M3, the STi is a complete package. EVERY single item in the car is upgraded. Just imagine how much it would cost to mod a 330i to M3 spec, I bet it would cost the whole MSRP of a 330i to do that. But like I said, if you are not looking for the complete package, maybe you just want super high power, or just good handling......etc. Then buying a base car and just mod the only thing you want would be a better deal.

Don't forget you can only mod the regular WRX so much until you have to upgrade the tranny and the internal of the engine, which are the major expensive parts. I am not an expert. But I think the WRX's tranny and bottom end is only good for a little over 300 WHP.

Simply put, it all depends on how much performance you are looking for. If you want more performance then the base WRX, you can't go wrong with the STi, it is truely a bargain. Even the M3, as expensive as it is, it is still a real bargain for how much it offer. Remember, factory performance is always cheapest with reliability that can't be match by any modified car. A few example would be the C5 Z06, 996 TT, Neon SRT-4, SE-R Spec V, M5.........etc. You get my point.