True the GTI seems to be more common nowadays compared to the Impreza but the Impreza engine is pretty much similar to the old ones right, especially if opting for 2.0, no? I don't know maybe you're right also. Choices for mods/tuning the Impreza should have more I think, rims also a lot more choices due to common PCD. Anyway that's just to give you a different idea. I know automatic Impreza is like missing the point but I think still a safer choice than the GTI. Furthermore, your brother already uses a GTI and your friends the E90 then why not be different instead of just follow everybody else? Like you said GTI also pretty common now on the roads (plus it looks the same like the TSi), so with the Impreza at least more rare/different, plus you'll smoke your brother and those 3 friends too at least in the straights.
Anyway it seems you've made your mind at least to about 80-90% like my friend last time, so I suppose not much I can say anymore. Just go ahead and buy, hopefully it turns out good for you.