Well, it's hard to compare sometimes. I've heard that pretty much all games released on both Xbox 360 and PS3 will look better on Xbox 360 because of the way they are ported and/or processed. On the other hand, PS3 exclusive titles have a much better chance of using the PS3 hardware more effectively and could potentially look much better than a similar exclusive Xbox 360 game.
Artica, There is DLC for PS3... but is there PS3 DLC specifically for Fallout 3? I'm not sure how old Gray Fox's source is or if it's quoted accurately but it's not impossible that DLC would be withheld for this specific game due to financial or technical reasons.
I'd say it all depends on whether you really want a PS3 for any other games. I certainly wouldn't buy a $300-$600US console just to play a game you could play on a system you already have. If your PC can play Oblivion well, you should be able to play Fallout on minimum. They're based on the same engine so the requirements should be similar, though probably a bit higher.
Minimum
System Requirements: * Windows XP/Vista
* 1GB System RAM (XP)/ 2GB System RAM (Vista)
* 2.4 Ghz Intel Pentium 4 or equivalent processor
* Direct X 9.0c compliant video card with 256MB RAM (NVIDIA 6800 or better/ATI X850 or better)