if you have the mod bug and can live with the smaller and generally cheaper(but not by much really) interior of the WRX, then you should get the wrx.
AFAIK, there is no aftermarket turbo options for the 5th gen LGT. you are currently limited to whatever stage 2 power levels are. also, the wrx comes stock with a bigger turbo, so you will get more out of it in general.
4-Time NNJR ESP Season Champion -15 ESP
3rd Place in 09' Pax Championship/Driver of the Year
2012 Northeast Divisional Champion in ESP