Fallout 3 for the single player experience, Borderlands if you're more of a grinding/co-op kinda guy.
Borderlands also has the worst storyline in video game history, whereas Fallout's is mediocre (maybe I was just spoiled by Fallout 1, which had a far superior story to 2 and 3 [fuck the GECK]).
Both are pretty damn easy in terms of gameplay, even if you notch up the difficulty to max in FO3. However, in Fallout 3 when you die you restart at your last save point, whereas Borderlands shoots you back to an autosave with the enemies usually at the same health as they were before. Only thing you lose is some money which is so ridiculously easy to obtain anyways.
Also, tons of exploiters in Borderlands, which ruins the game IMO. Reminds me of Diablo 1 with so many ridiculous dupe glitches, overpowered weapons, characters way beyond the level cap, etc.
Plus, Fallout 3 PC has the GECK (not the actual one in the story) which lets you edit levels, guns, etc.
I love both but I'd go with Fallout 3.