It's up to you. Either sprite would work, it's just what ever you like the most. That, and it would depend on how you map. If you map using doors that are 2 tiles tall and us the short chibi sprites, then I'm inclined to call you dumb. People do that, I don't know why. If you use short sprites, use the tiles that are made for them, otherwise you get a bunch of midgets walking through big-ass doors for no reason, lol. Anyways, it's up to you. RTP chibi sprites are easier to come by I suppose. I know there's a generator for the mack characters as well, but you'd have to edit them to make them futuristic.