Of those two, I like Arcanum best. Hell, I like Arcanum even more than I like Fallout. It has a great story, it offers you complete freedom and a humongous world to explore (the city of Tarant alone is bigger than all the cities and slums of Fallout 1 put together), the best damn character development I have ever seen (although some will say the exact opposite thing) and virtually unlimited possibilities (magic/technology). Great atmosphere (kind of Victorian), best soundtrack ever, but it does have its flaws as well: most of the npc's are crap and combat isn't very good, I suppose, very repetitive (but only when you use a melee character, methinks, technologically inclined characters can be very creative: poison, grenades, electricity, guns... + there is of course magic and boomerangs and chakrams and... ). Oh, and combat can be put to turn based, which is nice. Oh and you can travel by boat and train, you can take the subway in Tarant, there's lots of references to Fallout which will make you smile and *rant, rant, rant*
One more thing about Arcanum: there is an evil path that is much harder to play but does give you a different game experience. Worth trying it (second time through).
Planescape: Torment has a good story and a whacky world. Lots of reading to do, though, and the combat wasn't my thing at all. Nah, didn't like it, not really. Didn't finish it either, to be honest.
PLAY ARCANUM FIRST! ARCANUM RULEZ! (be sure to patch it, though, patch 1074)