Both these combinations cost about the same if you shop around.

Is the 755XB noticably less stable (Sony V1 on top)

I own the 525MVB legs (2 sets) with floor level spreader for use with my 501 heads (again, 2 of them). I would NEVER consider a tripod for video that did not have either a mid-level or floor spreader to hold the legs out and stable. Just my .02