Awesome looking Mazdaspeed3 and Golf.
I was considering an EvoX, a WRX Sti, e46 m3, 335i and 135i before I got my car.
I knew whichever car I purchased, I would modify. No question there. Both WRX, EVO, 335i and 135i are very easily tuned. The N54/n55 cars have a bit of a leg up over the STI/EVO because they are larger engines and are detuned from the factory. I have friends with STI/EVO's that are absolute beasts but they are not as reliable or comfortable as the BMWs.
I love WRX and EVO's but ultimately decided on getting a BMW because I didnt want to get a car I would grow out of. Every once in a while I have to drive clients and the bmw is much more appropriate for that.
Choosing a car out of the 135i, 335i and E46 M3 was a bit more difficult. I love M cars and had always wanted an E46 Since they came out. I decided against the M3 as it was an older car and lacked the warranties available on the 335i and 135i.
I chose not to get a 335I because of the size of the car. It is a gorgeous car but it is quite huge.
I ended up purchasing an 08 135i with 20k miles on it and 1.5 years of warranty. Almost 2 years later, the car has 36k miles on it and has been very reliable and had no issues at all.
I will be honest, the 135i has been a great fun car which has had tons of mod potential, but it hasn't been cheap to get the car to perform to its potential. Looking back, I probably would have been just as pleased with the e46 m3 and likely saved quite a bit of money in the end but I really have enjoyed the process of building my car and 1addicts is a very good forum overall with a solid community of people.
Coming from your GTI, you will not be disappointed at all with the 135i.
Both the N54 and N55 have their advantages. It really boils down to what you want to do with the car. If you want a dependable car that you just want to lightly modify over stock I would suggest an N55 car. They are newer which means most have quite a bit on their warranties. If you are set on DCT, then an N55 will also be the car for you.
If you want more mod potential and/or have more of a limited budget, then an N54 car will be a better choice. With the n54, you will be getting an earlier year car, which will likely mean no warranty and possibly higher mileage. So maintenance costs will be higher. The n54 has a ton of tuning support which for many on here makes it the only choice.
You can't go wrong with either really.