I came from a highly modded 2004 wrx. Great car, but the gearbox's are the weak link.
They are fairly plain looking in the inside and a bit tin-like.
Regarding their performance - unless you launch a wrx from 6000rpm, it wont get near the skyline from a start. They just bog down and go nowhere quick. Unless you want the gearbox to break, you won't end up doing many high rpm launches. Once in gear and rolling they do go very well once above 3500rpm (thats with the 2L, and maybe 500rpm less with the 2.5L).
Regarding fuel economy- the wrx drinks fuel just as much as the 350gt.
If you want to mod the car, I'd go the wrx, as it ends up being much cheaper. If you are keeping them stock, then the 350gt.