Depends on what you are looking for:
Planescape has a very intricate plot, and pretty compelling story and characters. But because of this, you will do a LOT of reading. The combat is not that big a deal in it. IT does have a strange, nearly Steampunk feel to it. If you liked the movie Memento, this game shares a lot of common elements with it. Gamewise, the engine is a little clunky, but then again, by the time I played it, I had played later versions of the engine, and so it seemed awkward to me. If you haven’t played any of the other Infinity Engine games (Baldur’s Gate, Icewind Dale, etc) you may not notice any problem. Planescape has a fairly linear plot, but you can make choices that determine your eventual outcome. I know of at least two different endings, and I think there’s at least one more. There is one ending, the one I got, that does not end with a big knock-out battle with a huge “boss enemy” which appealed to me, since I think that’s usually a dumb way to end a game.
Baldur’s Gate is more straight-up D&D. The plot is pretty linear, and some people have found it very confusing, but that was not my experience, and there are plenty of walkthroughs online to help you if you do get stuck. The plot was nothing special, typical fantasy stuff. There’s an expansion to this, but like all the other expansions for these games, it only adds a few more weapons, some spells, and an INSANELY difficult battle. It’s built very much on the idea of “bosses” with a set of combats all leading to bigger combats. I tend to not usually like this sort of thing, but it didn’t really bother me that much. The combat is sort of real-time, but you can pause to change what your characters are doing, which I REALLY liekd because in general I don’t like real-time combat. I found this system to be a nice happy medium.
You didn’t ask, but if you like the combat aspect, Icewind Dale is even more combat-heavy.
All of them have nice graphics, great music, and I found a lot of fun. The first two you can pick up for about $20 each, tops, so you might not have to choose. If you plan to play both, I might suggets Planescape first just because if you play BG first, you might find the Planescape controls clunkier, like I did.