I heard a Filter concert on a "Sunday Night Live" show on a local rock station. THEY SUCKED LIVE! I've been to a Tool concert, and they are one of the few bands that actually sound better on stage than they do on CD.
I own both Filter albums, as well as all Tool albums except Opiate. I find tool to be a much more musicly complex band and much more enthralling.
Filter is as another said, a strong basline with a fairly linear sound. They are good for listening too in the background of a video game or a movie, but by them selves, they are not as captivating as tool.