I just got TOCA Race Driver 2 the other day (with the $10 MIR offer), and it's pretty good so far. It has a pretty large variety of race modes, but the drawback is the lack of cars. There's probably 40 or so for the entire game, which means theres 1-3 cars for a specific series. Still, it's a pretty good game for those waiting for GT4.
Now, comparing TOCA RD2 to PGR2 is tough, since I just got TOCA RD2 yesterday, and I've been playing PGR2 for the past month. PGR2 has a lot more cars to pick from, and the style of racing IS different from Race Driver 2's. It's not really a racing sim per-se. So if you're looking for an arcade-like racing game, go with PGR2. If you want a racing sim, go with TOCA RD2. If you really don't care about the type of racing you participate in, and just want an addictive and challenging racing experience, go with Project Gotham Racing 2.
BUT, here's a thought. Get TOCA Race Driver 2 now with the mail in rebate voucher offer from EB, and use that voucher to knock down the price of Project Gotham Racing 2 to $19.99. The rebate offer for TOCA RD 2 is time-sensitive, but the price for PGR 2 isn't. Something to consider.
Both games are pretty good, but right now, I would say that Project Gotham Racing 2 is a better pick, just because there's a lot more skill, precision and challenge involved. I've put in over 40 hours into that game already, and that's just offline. I haven't gone online with the game due to the lack of a broadband connection.