I planned to make a conversion, but am currently GM'ing a D&D4E campaign and playing in a SW Saga campaign, so it will have to wait. On the other hand I'd like to use the actual D20 system I have the book for, which has the right flavour already and need no conversion work, but there's also cons against that.
First, I would need to learn new system. While basics are there, each D20 has it's specialties. Second, players would have to learn a new system. This needs some amount of dedication due to all those special abilities classes have, feats, and especially teep powers. As I'm thinking of the game as an episodic game where character deaths/changes would be inevitable from time to time, I believe preparing new characters would need too much time, and would be difficult as I'm the only one around with the book.
True20 would have advantage of certain solidity, and several people here has it. Characters would be fast to create. But, after playing 4E and SagaE I belive True20 is outdated too. Consolidation of the new games seems so much better, and I have to say I really dislike true20 powers/fatigue, as they are all different. If I had time, I'd use existing resources to build a whole new hybrid -system for B5, having some elements to bring flavor (but being more consolidated than in B5 book), elements that make both character creation for PC's and GM easy and fast. New systems are too heroic for B5, though, as is True20 by default.
One of the coolest systems in True20 is damage saves, but as saves feel already outdated, I guess they should be turned into attack rolls too. Counting how much character saved by also felt clumsy sometimes.
Btw, what's changed in Revised edition? I haven't followed what's happened with True20 lately.