Well, I tend to think that SSX3 is better. I have it for my PS2 and I bought it the first day it came out. Now, I am an extreme Nintendo fanboy, but online multiplayer took precedence and it is awesome to play online. Albeit you can only play against only 1 human opponent, it is really fun. Graphically, it is really good, and I am not having framerate problems. BTW - I don't think you can tunnel SSX3, but maybe with 1080, but I am skeptical with tunneling
1080 looks kinda good, but I am worried about the longevity, since it looks a little slow and boring. Plus the levels don't seem that big looking. So it may be a bit more fun than SSX in the short run, but not for longevity. I'll rent it to make sure of it first. However, I highly recomment, SSX3 if you have a PS2, but as for a GC maybe, but I think online is really needed to fully enjoy it. But Tricky didn't have it and after renting it 4 times I bought it lol. PLus I bought SSX used for 10 bucks and I was willing to pay full price for it.
At worst, just wait till 1080 comes out and make a descision.