Well, I can't answer for the other two, but my brother had the 3250, forerunner to the 5700, and it had great sound clarity. It was a great phone all around.
Like char777 I can't tell you which is best. But I've been using the E6 for a while now and it works well as a music player. Sound quality is decent. Volume is good but could be better. However, since it's a linux based phone you can modify the volume table and essentially have it as loud as you want. If you do choose the E6 check out motorolafans.com. There are instuctions on how to make your E6 quadband and Edge capable.
