I'd go for neither...for me the entire BG saga is the best, BG2 on its own is good but not complete without BG1+TotSC and ToB as well (at least in my opinion).
I might give Planescape another go soon, but I struggled getting into it at the start as it was so different to any other game I have played (not in terms of the engine used etc., just that coming from other games on the same engine I just found it really hard adjusting to the gameplay in Planescape - for example, playing for over an hour and not having one battle at all. At the time I could not get my head around that and so gave up with the game after a few hours of playing it). I love the story elements of games, love the role play elements of games, but I also enjoy the fighting side of them too. However, I've matured a bit since I last gave it a go (well got a few more gray hairs anyway lol) so maybe its worth another shot and who knows, I might end up preferring it to the BG saga
