Agreed.It is difficult to draw a hand to hand comparison. In my experience, MC tend to be more picky at the initial stages (lots of people kicked out at the application stage), whereas the US firms tend to be more picky at the later stages (lots of people kicked out post-VI, AC etc.). US firms also look for different things - quite a few value practical entrepreneurial stuff more so you're more likely to progress with them if you've done something to demonstrate that.
This is all speculative by the way, just my own personal thoughts and anecdotes.
Speaking from this cycle's experience, whenever the US firm has given me more opportunities (a longer application form) to expand upon my life experiences it's always been a positive outcome to the next stage.
MC firms have been outright long-holds or rejections. (with the exception of Ashurst and it's v v easy-going word count)
I've been vastly more successful with US firms this cycle - I suspect, because of the fit of candidate they're looking at. My grades (2:1) might not be stellar, but I've been able to back up my traits with plenty of life experiences.