Depends on how much you want to spend. I like Babylon 5 a whole lot more than Lexx, but I still enjoy both.
Assuming you're in the US, you can get the entire series of Lexx for $20 at Target. I don't mean season, I mean the whole thing.
They were originally released one disc at a time, then repackaged as season sets (at considerably higher prices). They've been picked up by another distributor and priced extremely cheap. You can get a box of seasons one and two, or a box of three and four, for $10. So, $20 for the whole series.
There are no additional movies with Lexx, that's the whole thing.
B5, you can get the full five seasons for $50 from amazon.ca ($9.99 per season). There are also several movies and a spin-off that tell other stories in the same universe. They're like bonus stories. Not needed for the series itself, but fun extras you can use to spend more time with your favorite characters.
Edited to add where you can get Lexx.