Take it from someone who's been playing with the 32bit client for years and the 64bit client since it was first released around October last year:
64bit is the better one. I was playing for three weeks straight, average eight hours a day, until I literally got bored and tired from playing the game and there was no single crash ever. The performance difference is nothing but groundbreaking. In the over ten years I've been following and playing the official version with the old 32bit client, this has never been possible at any moment.