Planescape: Torment has inarguably the better story, which is among the best in the gaming industry in terms of intricacies and substance. It also boasts an extremely memorable cast of characters with some fantastically well written dialogue and emotional depth, again some of the best in the industry. So if it's a narrative experience you're after Planescape: Torment is the one you want.
It's not without flaws however. The combat, which mercifully there isn't much of, is pretty mediocre at absolute best and well over 90% of the game takes place via dialogue, so you'd better enjoy reading. It's also quite a long and unforgiving game at times which is why a lot of people tend to have trouble getting into, though I always encourage them to keep trying.
So brilliant narrative experience, probably one of the best in the gaming industry, but it's also more of an interactive book than an actual game. Visuals are alright though once the widescreen mod is added and the soundtrack, mmm,
great soundtrack!
Arcanum is a different sort of beast entirely by comparisson, and not without it's own comparable strengths and weakness. In terms of narrative the actual story isn't nearly as good as that of Planescape and the game has relatively few memorable characters
HOWEVER Arcanum makes up for much of this by being one of the better games out there for player interactions. What do I mean by that? What you play and how you play them can make a pretty big difference in the game, ensuring no two playthrough are quite alike.
So it's not as complex or emotional as Planescape, but it is more malleable and interactive. Gameplay is... meh. Not terrible, not great. It's more Diabloish than anything and it doesn't do that terribly well, but still its serviceable. Can't say the gameplay ever really made me want to stop playing, but it was never the driving force to continue either. Same with visuals; not terrible, not great.
Again, neat soundtrack though.
Both games actually have really unique and fantastic setting, but while Planescape's is a ton of fun to explore and probably sticks with you longer, Arcanum's does have the advantage of letting you explore more of their world and letting you try more things. Can't think of any other game out there save maybe Fallout 2 that gives you quite as many options with character creation and play. You really can do and be just about anything in Arcanums setting, from a monocled elven wizard to a Dwarven mechanist to a crazed prostitute with a flamethrower... probably one of my most unique rpg characters to date. Helluva lot of fun though!