As to which to watch, B5 or Lexx - well, they're two very, VERY different shows!
B5 is a 5-season-long novel, pitting humans against godlike forces for control of the galaxy, with many wonderful stories of personal and interpersonal struggle along the way, and epic, galaxy-shattering events. Plus, ya know, lotsa cool space battles. Keep in mind, at the beginning, that the CGI is some of the very first CGI special effects ever used on TV, so it starts out a bit crude. By season 3 it's state-of-the-art for the times. Also, season 1 is almost all set-up, like the early chapters of a book. Some people think it's kinda dull, but each episode has a little plot arc clue that may pay off several seasons later. It picks up the pace in Season 2, then everything explodes in S3 and the ride gets wild.
Lexx? well, I started watching it when it came out, thought it was insane over-the-top softcore porn, and gave up on it.