After playing both, I'd recommend your initial course of action - buy the 360 version and double-dip with the PC version later.
The 360 version actually plays very well; its menu is almost identical to its PC counterpart and multiple ways to change weapons makes it relatively easy to use with the gamepad. Is it superior to the mouse and keyboard? Hell no, but it's not bad. The game looks great too. To run it 360-level visuals on a PC, you'd need a rig better than mine (mediocre), which I know is substantially better than yours. If there's one thing Valve got right (well, Episode Two looks pretty good too) in this port, it's Team Fortress 2.
The maps max out at 16 players, which made me itching to go to 24 player maps on the PC, but I tell you what, I actually prefer 16 players after playing both. TF2 feels really well balanced and competitive at 16 players, and more like feeding frenzy with 24 players.
The biggest gripe with the 360 version is peer-to-peer service on XBL, which led to many laggy games last night, versus dedicated servers on the PC end. I really hadn't had a problem since and after several hilarious and awesome matches with friends, I wholeheartedly recommend TF2 on the 360 as a viable alternative if you have an underpowered PC. The stat tracking that's on the PC is also on the 360 - really, much of nothing got watered down in the port except the way you play (peer-to-peer) and maps maxing out at 16 players (a positive, in my book).
For the long-term investment, TF2 is a no brainer on the PC. It will have a dedicated community, free maps and a more competitive, team-oriented player based. TF2 on the 360 is a ton of fun, though. If you need a quick fix, it's a good buy. And removing the comparison from its PC counterpart, it's easily one of the best multiplayer games on a console. Hands down.
