I haven't played either, but I have seen comercials for both. I would say that it depends on what you like. It looks like they both are 1st person shooters, and I bet they both have on-line play, but fallout 3 is in the future, and COD: world at war is in WWI.
~Binary
I personally am a COD gamer, so I'm getting COD world at war for christmas
both of them are great, and both of them you can blow people's limbs off, and evenly matched, too. cod5 has the online and the flamethrower, and fallout 3 has the choice of weapons