I personally prefer last.fm for discovering new music but what's your favorite? If it's similar but different please specify.
I had no idea what Last.fm was until I saw this thread. Before I downloaded the last.fm application I woulda said Pandora but seriously Last.fm is fucking awesome.
Pandora usually has a tendency to get into really terrible music after awhile of being on one station for too long. Friend showed me last.fm awhile back and it's all I use now.
Maybe it depends on what kind of music you like. I listen to Bonobo, Sound Tribe Sector 9, .. type of stations a lot on Pandora and hear a lot of sweet tunes. I haven't tried Last.fm yet.