I had a 99 Disco II that got knocked into a river in a rock slide and everyone walked away without a scratch. The truck was totalled due to bent frame and rippled firewall. Before all that it was fairly reliable (for a Rover) the tough fixes were things like brake wiring wearing out and not letting the transmission out of park, that took about 2 weeks to figure out. But it never over heated, my wife loved driving it, the dog loved riding in it, and it was a lot easier to drive than my current Series III. But the Series is a lot easier to fix and wrench on in general, plus it has more character and you can find every major part to rebuild one, that may not be the case with Discos. Of course Discos have amenities like radios and door seals.
I would probably just get one of each and call it a day.
1973 Series III, 109, Ex-Mod
1999 Discovery II, Cataclysmically Lost