I will agree, I own an 08 STI and am looking to get another car or possibly replace the STI with the 135i. The 135i definately has the speed up top. Stock for Stock from a dig the cars will be neck and neck with the STI just slightly up front for the first 80-100mph and then the 135i will own.
Around the track its a toss up. There have been some articles like C&Ds shootout that have the STI being very slow. C&D is hardly a magazine you should take to heart. A problem with STI magazine times is the fact that to get the most out of them you have to drive them differently than you would any other car around the track. I think a lot of test drivers do not realize this and just scream understeer!
Mod for Mod the 135i is going to respond a little better.
Stage 1 (just tune) STI you are looking around 270whp and 300wtq, Stage 2 (DP and Tune) STI you are looking around 300awhp and 350lbtq somewhere in that range.
The awd will always have the advantage from a dig and the 135i will always have the advantage from a roll. Both are very solid cars!
Comments on interior quality, the plastic in the 08 STI is a step up from previous years but it still feels like an economy car. The seats are alright but the driving feel is great and the car is nimble for its weight. The interior quality in the BMW is better, I don't think that is in question
I would probably lean towards the STI in terms of reliability though. and the 6spd is pretty much indestructible. There are STI's running 500whp on the stock transmission with no issues.