Stick with Fallout 3 and get the Game Of the Year edition, since it has all of the DLCs, two of which are a must have anyway. Fallout Vegas, IMO, is a joke and I wouldn't hold it in the same breath as Fallout 3. While Vegas has some cool moments and a neat music score, the story and quest-line is all over the pace unlike the more straight-forwardness of Fallout3. The fact that certain quests will fail when you do another aspect of its related quest, that also entails achievements, makes the game seriously frustrating. You literally have to play out 5 different endings to get their respective achievements, and certain other achievements are tied to those as well.
The graphics are nice but the game uses the same engine as Fallout 3. The character's faces look worse than in Fallout 3 by far, although the modeling is the same in movement and action. Fallout 3's characters never looked that great anyway, so expect worse in Vegas. Trying to choose perks is even worse as there are now so many, the decision on which to use it maddening. For example, In fallout 3 with the edition of a certain DLC, you can reach a max level of 30, same as Vegas. But as you level up in Fallout 3, you can choose a perk each time. In Vegas, you only get to choose a perk every other level, meaning you can only get 15. They added just about every perk from Fallout 3 to Vegas, meaning you now have nearly 90 perks to choose from.
If you do get it, be prepared for nasty bugs and glitches. Yes, Fallout 3 has them but not on this scale. Again, IMO, get Fallout 3, it is the best of the Fallouts by far and the side-quests (which operate outside the main quest-line unlike Vegas) are even more interesting.
[Edited by tommy6860, 11/4/2010 2:54:37 PM]