The MT800 rotor is XT, not Ultegra. The Ultegra rotor is RT800.

The main difference between these and the RT70 rotor you have is weight, and cooling ability. Both the MT800 and RT800 have additional cooling fins in the center of the rotor that help dissipate heat. There's no issue with running different rotors on different wheelsets, but you might need to add shims to avoid having to re-align your calipers each time you swap.

I currently swap two wheelsets on my CX bike. One set has MT800 rotors (centerlock) and the other set has RT56 (6 bolt). With shims they both work fine, no adjusting needed.

Edit: I should add there's very little difference in braking power/feel between the rotors, but the XT rotors do feel a little more "grabby". This also might be because they are on much lighter carbon wheels, whereas my RT56 rotors are on heavier alloy wheels.