If it was shot on film with
If it was shot on film with "practical effects", i.e. miniatures and animatronics, I think I’d opt for the Blu-ray release, but stuff like LEXX was shot for TV, with CGI that was probably not rendered in higher resolution than necessary for SD TV broadcasting back in the late 90’s. I’m not sure, but I think they probably used digital or video cameras rather than film stock for that one. Seems like the new Star Trek: The Next Generation blu-rays are fantastic, due to the fact that it was shot on film and mostly used miniature effects, though there was some use of low res digital editing and compositing that made the TV prints unusable for bluray, but as they had the film stock they could recomposite and re-edit that with HD quality effects again.
I’m not sure about Babylon 5, looks to me like it was shot on film (I could be mistaken though), but since most of the effects were CGI, and they’ve lost the original files, they’d have to recreate and rerender all that from scratch again. Sounds doable to me, but would probably be much more expensive than the process was for TNG.
Even if it was shot on film with a lot of practical effects, like most of Dune was, there’s the risk that they botch up the transfer from film to HD too, like I feel like they did with this Dune release, so I’m not sure there is much general advice I could give.