i ran a vf34 on my '05 wrx,my original target was 350/350 and on the research i done at the time
seemed to point at the vf34,i will add that most of that research was done on nasioc.the spec of the car was;
perrin fmic
injen cai
650 injectors
walbro fuel pump
full 3"meercat decat exhaust
decat up-pipe
avcr
after remap the car achieved 340bhp/340lb ft.8 weeks later the car was mapped with 10% methanol and achieved
370bhp/348lb ft,car was ran every day for work and made at least 60 crail launches without problems.the '03 wrx
gearboxes will take a bit more power than people think,kevametals car was running circa 420bhp on a wrx.at the end
of the day both turbos will be sufficient for what you want to achieve,as andy said the 35 will achieve full boost a tad
earlier
jim