I'm no production exec, but i'm very much more in favour of miniseries.
In ANY long-running TV series, you get a number of episodes which should never have been made - waste of money time fillers. (OK, I have a hard time thinking of a B5 episode I dont like however.....)
With a miniseries, you have absolute control over the plot, cast, etc in the same way as you would with a movie. If you plan a set of miniseries, you can double your benefits!
Sales wise, I suspect that even though the B5 DVD has sold *very* well, other DVDs of TV series are not doing quite as well (what does that say?!) however buying a mini series is a far lower investment of cash and time so sales should generally be higher.
I have seen mini series done very badly - one hour worth of story turned in to 3 - but Dune, Children of Dune, Dinotopia and many more show how well it can be done.
Having said that, how about making mini series for the big screen? Lord of the Rings proves it can be done *very* well indeed, and the three-hour format is becoming generally accepted once more (ahh for a return to the era of the epics!)
DVD-wise, the B5 DVDs are approaching perfection - just the right level of Easter Eggs, extras, commentaries, just the right price, and all contained in a single almost VHS-sized box (I used to own ALL the videos - they are on indefinate loan, while I consider renting out the shelf space they used to occupy