I have to agree with the previous two posters. I own all of JC and I love it, however the problem always arises that what do the players know. I had players that would read what they need (ie the core book and whatever supplement they are from ie a Marsian PC would read the Mars book). However the majority of them won't, and even worst, though maybe one and two is willing to sit down and listen to you talk till 3 AM telling the story, many would not want to even do that. Lastly, those that would look at the book to just see the kewl powerz. Also the Sil system may be a bit hard to get into if you do not know that much. And personally I think the Sil system is better design for Vehicle combat, for character interactions the system is a bit loopy, acceptable but loopy.
Now B5 has a detailed and rich history too, one that doesn't require you to get every supplements. The supplements out reinforces and fills in the gaps to an already existing souce material, the TV show. It has more fan sites to find extra info, and uses D20 a system that is easy to learn and not so loopy (as long as you get over the character levels, but if you do not mind character levels then it is fine). Why talk and describe when you can turn on the TV and pop in a DvD or cassette. You can not buy the Narn book, not such a big problem because there is enough websites out there and the TV show itself to let you know everything about the Narns.
But I imagine you want a thematic response, well there is no better theme. It all depends what you want. JC has its own theme, which is sumarised in the first chapter so I need not to repeat it here. B6 has its own too, just watch the show and you will know it. However the B5 Universe is big, other themese can easily be explored depending where you sett he game. And here I think is B5 strong point, it is a Huge world.
What does B5 have that JC doesn't: Telepaths, Techno-Mages, fully developed Aliens each with its own culture and style, Ancient races that play a galactic chess game, organisations that span across the Horizan like the Rangers and Psi-Corp, a cast of already made NPC that everyone has learnt to love and had time familiarize with (if you are willing to play the game during a timeframe and area where you can meet them), and large multi-raced operatic space battles. What does JC has that B5 doesn't: Space Battles that follow hard-core Newtonian Physics and big anime mecha. JC is a microcosim of Solar system in turmoil that can be epic, B5 is a macrocosim of a Universe in turmoil that is already epic.
I might love JC, but I love B5 too and the truth is from a GM perpective B5 has more story hooks and it is easier to come up with plotlines in it. This is because you are dealing with a vaster world with more unkowns, you have more to fish from and more room to insert totally new things.
