I suspect that we'll see Nicola on the next one, as Peri spent more time with Six than Five, I think (and she still looks damn sharp, IMO).
Well, the only Sixth Doctor serials she wasn't in were the last two of the overall "Trial of a Time Lord" arc, so I doubt they'd show them. It'll probably be something from Six's first season. I think my vote would be for "The Two Doctors." That's probably his best, even aside from the Second Doctor's appearance (it is by Robert Holmes, after all).
I believe that story has a lenght of 3X45 minutes. Probably too long.
My vote would be for Vengeance on Varos, easily his best. Two Doctors really drags.