It would seem to me to take out the best of each. So far it seems that B5 has nothing in regards to fleshing out the land based military compared to d20mod/future. Power armor for one could be possible for some of the higher tech races. Some hint for cybernetics is hinted on Babylon 5 and could be allowed as well. Tho one weakness is that its possible for a mod d 20 to survive a 50 cal hit <yes there is even max damage rules that can get a character to -1 health> , when in reality even a near miss with a 50 calibur round can maim or rip a person in half. Babylon 5 has pretty good idea on how to design space ships and arm them, with the ship builder book.
I would love to have a babylon 5 book expansion that would flesh out these matters, but untill then d20 future and d20 mecha builder book <by david pulver> work to help build tanks and other futuristic land based military vehicles.