I would personally prefer a series, and mainly for the reason Dukhat states.
I liked The Lost Tales, but it was lacking something, it seemed to lack the magic of Babylon 5, perhaps due to lack of Londo, Garibaldi, and especially the late lamented G'Kar and Franklin. It also felt small and cheap, something Babylon 5 had never felt.
The major difference between a TV series and a few movies is quality. If you have a slightly weaker outing in one episode, it can sometimes be carried by the others if the quality before and after were great. People didn't turn from TOS in droves after "Spock's Brain", they were just never there to begin with thanks to the network's burial of the series in the graveyard slot.
If it fails in the second movie of a group of three, it may damage perceptions of the whole.