Having had an STi, stock, power doesn't happen until 3,500 rpm. I heard you can program the engines and start getting usable power at 3,000 rpm.
From my previous off road experience, that sucks. Need a little more oomph at the low end. I just can't imagine having the rev the crap out of the engine and spin tires all over the place, trying to get up a hill. There is no way that can't get your blood pressure up.
Also if you want to start talking about relability, forget the STi engine! Despite being stock, I had that car at the shop a few times to get various whatnots replaced. Also needed a clutch at 30k. 2nd gear was starting to get wacky on me (before 30k on the car) and apparently this is a common problem with them.
Flipside, the 6 speed was aaaawwweeesssooommmeee. Center lockable diff was kinda stupid, I always selected "open" when on normal pavement. Gave me 2 more mpg. However when traction was low, it was like the fun-o-meter. Dial it back to 90% rear power for more fun. Front and rear LSDs were clutch as well.
So yeah, get that 6 cylinder engine, bet it has decent power throughout the band, won't suck fuel miserably. STi for pavement and have fun.