the apexi is 10x better at filter then the blitz, but the blitz pulls in more air (go figure as it is not technically a filter, but a SS mesh). Any short ram will do you right. I love my Perrin, but I will probably get Apexi as i dont want to have to reoil and crap (that and Apexi was the BEST out of HKS/K&N/SUS/Apexi filtering wise).
The Apexi is a great quality intake for the Sti. If I had to do it all over again, I would actually delete the resonator and just put in a drop in performance filter to get the same power gains. The stock box is not restrictive, the resonator is. The real power is made by adding a performance filter and then tuning the fuel and ignition curves together. Our ECU's are too sensitive to performance upgrades.
I have been in the market for a shorty for a while now. I finally decided to get the Blitz today. I have read reports over on www.ClubWRX.net about Apex'i and lean conditions. I "HAD" a TXS shorty, but had to return it as the mass air flow sensor didn't fit on tight. According to TXS, it is a manufacturing problem they had with a recent run. They were supposed to have this buttoned up around the middle of February, but this has given me too much time to think about alternatives. I looked at Perrin, Blitz and Apex'i. I think Perrin in the best bang for the buck with the silicon inlet hose, but I dislike the oiled foam filter (more maintenance than I would like). I was ready to buy the Apex'i until I read posts on ClubWRX. Seems like there aren't any complaints on the SUS that I could find. It seems to be the shiznit, but it does cost more. What the hey, it's only money.