OK here's the deal with both games (personal opinion, of course):
Tribes 2 -
Very, very fun. I like it. I think it's a very deep multiplayer game. It has tons and tons of weapons and gadgets and cool vehicles, and it really creates a nice large-scale virtual battlefield. Some servers can handle 255 players duking it out at the same time.
It's not TOO rough on a 56k-er, either. I used to play it on my old modem, and it was decent. Much better on broadband now (just like anything else).
There is one thing you gotta watch out for with Tribes 2: You have to have a really good system. I'm talking AT LEAST a Pentium III with a GeForce 2 card (NOT the crappy mx200, it won't be good enough) or a Radeon, and 256 MB RAM. 128 MB is really choppy. A TNT2 card might work. Voodoo 5 users are out of luck, the game is really harsh on that card.
You can play with bots. They're OK, but they eat up your system resources like you wouldn't believe. On my system, which is pretty good (see below), I can't add more than 10 bots without my framerate slowing down too much. However, if you're on a LAN with 3 people, and you add 6 or 7 bots, you're set.
Conclusion: Tribes 2 is a blast, if your system can handle it.
NOLF:
This game is AWESOME. Except for the multiplayer. The multiplayer sucks. Just pretend it doesn't exist and you'll be much happier.
As for the single player mode, it's great. It was named game of the year by tons of people for a reason. It's a blast to play. The gameplay is exciting and challenging, and the story is great, and the game itself is absolutely hilarious.
Conclusion: It's like Austin Powers, except it's funny.
Buy both of these games, and if they're not in stock, prepay for them.