Where to start, really. They're both as crappy as it can get, if you're in the market for a "hot hatch".
The Golf is a pig, for starters. A GTi weighs over 3000lbs, while the Civic weighs around 2700lbs, give or take a few pounds for both. This is a very noticeable difference, especially around a racetrack. And the aforementionned R32 weighs nearly 3400lbs. It'll get outpaced by the EVO and the STi, which are priced similarly.
The biggest problem with both cars reside in the suspension/brakes/tires department. Seems like both manufacturers decided to equip these cars with rubber designed for a Buick. First step would be to get some stickier, more agressive rubber, with some lighter wheels than the lead blocks found on both cars. The aftermarket has good option for both cars when it comes to suspensions/brakes, too.
The Si feels more like a driver's car inside, with grippy seats, and a well positionned shifter, whereas the Golf's seat are, IMO, pure crap. They don't hold well, they're slippy, and more importantly, they're freaking heavy.
Engine wise, you can always swap the Si's pile of crap for the RSX Type-S engine, which has a better aftermarket, but that's a lot of fiddling around. The VR6's been tuned for years, and it's a good engine, but I never liked the 1.8t, which just seems too fragile.
So, in the end, you're kicking a dead horse, if you choose any of those two. You'll likely be spending way too much money on them to get them to a level of performance where they should be anyway. Have you thought about the Biggie Cooper S? It's pretty potent, if you can get used to the whine...
Since you want to tune up your car for the racetrack, have you thought of a slightly used car? You'll be less likely to deal with a very long warranty period, for one, and you can get pretty good deals. Something like an Integra GS-R, an earlier, 99-00 Civic Si or a Neon ACR (if you can find one) will make a very, very good track car from the get go, and with the money you'd save from not buying one of the two piles you mentionned above, you can get a pretty good amount of go-fast stuff. Heck, you can get an Integra Type-R for less then 17 000$, and that's the best handling FWD car ever, even better than the Neon SRT-4 (which, lets face it, gets most of that praise because of the stupidly good engine). And I don't say that because I've owned one.
