first of all, what is "medium-frame"? lol. a crop body or a medium format body. if its medium format, then i think the 5dii is close enough in pure resolution to an MF camera that its not worth re-tooling your entire setup. on the other hands, there are things that simply cannot be matched by a 35mm size sensor nomatter how many pixels you cram onto it. low light high-iso is horrible on medium format cameras, idk why that is because delta 3200 on MF looks waaay better than delta 3200 on 35mm but i guess that hasnt translated to digital yet. tonality and shadow detail and dynamic/contrast range are simply better with medium format, noone will convince me different in this day and age.
it all depends what your uses are and how much money you are willing to spend. if you're passion is landscapes then no, a 5dii will not match up to a good medium format system. but be prepared to sell your kidney.