Lexx? well, I started watching it when it came out, thought it was insane over-the-top softcore porn, and gave up on it.
There's more to Lexx than that (although that's definitely
part of it). It's best episodes are bizarre surreal farces - a character playing a game of chess with a Death-like antagonist, the cast being compelled to sing in a musical about the undead character's living days. At it's worst, Lexx is the worst TV show I've willingly sat through, but in its better hours, it was pretty fun.
But to answer the OP's question, Babylon 5 is a better purchase. Lexx has it's moments, but it's wildly uneven and it's probably too weird and/or too filthy or too often terrible for most viewers. And even then, when it comes to outre space opera, Farscape is by far the better series.
Babylon 5 has it's own share of problems - the first season is of decidedly mixed quality - but it when it gets going, it has some of the most satisfying arced narratives in sci-fi TV, and in Londo Mollari, one of the genre's best characters.
I hope this helped.