I think in cases where the EF adapts especially well, almost seamlessly, it's worth the savings and being able to get something sooner. To me, end results count, and if ergonomics are good, it makes the decision easy.
I'm probably going to end up getting rid of my ef 70-200mm f/2.8L IS II sooner rather than later. I just can't use it on the R comfortably in vertical position/portrait orientation. The available grip won't help either as my thumb would be too far from the upper right corner of the display screen for touch-and-drag.
Only thing holding me back from selling it is, well, ok two things...First, it's a superb copy (and paid for!). But also, I wonder if better ergonomics on the next body might "correct" some of the front-heavy issue I'm having.