Just objectively speaking Baldur's Gate 2 is the superior game. Better gameplay, more cohesive experience, more interesting companions, better visuals and so on. The story is excellent, the tactical rpg combat is fun, and it's got a great soundtrack. Honestly as far as rpg go it's pretty much the high bar to which I hold up every other rpg experience. So far none have toppled it in my eyes, not overall anyway. Every now and then another rpg might do one or two things better but I still find BG2 the most cohesive of the genre.
That said if you're into the steampunk setting, love the Fallout style of wackiness and 'everything goes' roleplaying, and prefer your combat a bit more actiony then Arcanum is a damned solid experience as well. Terribly balanced even with the various fan mods but then I've always felt the novelty of being able to play as a hooker with a flamethrower outweighs that. Not many games allow that sort of over the top insanity when it comes to character cusomization and roleplaying options.
