Bluebird,
There are several things the 550 has over the Roland. One is editing styles.. You can edit the preset styles or create your own from scratch with the 550... Plus the 550 has 16 dedicated track buttons on the face panel.. Makes it so much easier to mute tracks on the fly. I also think the 550 has a better display too. One thing I wish the 550 did have is the joystick rather than the wheel. Also the Roland has an internal sequencer. The 550's is direct disk, but it does however offer quite a bit of editing for a disk based sequencer. As far as sounds go, that's really subjective.

Squeak