I really liked B5 and feel that it was an excellent show. That said, if I had to pick between B5 and DS9, I'd choose the latter.
B5, while mostly great, was a little cheesy at times.
The whole "ZAAH HAH DOOM!" thing was a bit silly, and the Vorlons were about as impressive as a prop from an Ed Wood movie. In terms of production values, the show looked like 1980s-era Dr. Who. BG came out only a few years later, and think about how much better it looked by comparison.
All complaining aside, it was a great show with a great cast and it had some some real compelling stories.