The 5D II doesn't have significantly better IQ than the t2i (which has the same IQ as the 7D). They intercut fine. The big advantage (or disadvantage if you're an AC) is the option for fast wide lenses and shallow focus. The image looks exactly the same to me otherwise. The GH2 DOES have significantly better IQ (as do the C300 and F3 of course!), but lacks latitude and the small sensor makes lenses tricky.
But the 5D is a half stop slower at the same ISO for whatever bizarre reason. So don't underexpose! Oh, and the technicolor profile is GARBAGE except if you're shooting on a show that's posting in log and you need log footage to match. It can really strain the tonality of your mids if you aren't careful. Neutral is so much better! It is harder to expose properly with technicolor and you have to use a light meter or expose in another picture look profile and switch back before recording. But it does help a bit if you're using a dSLR as a crash cam and need flexibility (but you might also need to do NR and skew removal so it intercuts!). I don't really think it serves much of a purpose but I did use it on a log show once and it did fine.
You can watch some of what I've shot on a mix of heaps of Canon cameras and see for yourself that they all look the same (contingent on grading, of course):
http://vimeo.com/37024225
0-11 seconds is t2i (with film grain added)
14-26 is 5D II/7D mix (can't tell the difference? neither can I)
31-39 is 7D (with film grain and looks like ****, why did I include this?)
39-44 is t2i (with film grain)
54-1:00 is t2i
1:01 is 7D (and ugly)
The rest is bad and is red and hvx from like six years ago... I look at this now and it's no wonder I couldn't find work with hvx footage on my reel and so much bad material. All the decent stuff is from the only two gigs I got in the past two years and a personal project.
Why does 14-26 look best by far? A genius director, good stylist, location, and model--and good lenses. Camera is secondary. I do find the 5D III to be noisier in the shadows at low ISOs and cleaner in general at high ones (by a lot), but otherwise (excepting its lack of aliasing) its footage looks exactly the same, too.
That said, the Alexa is totally magic. Hopefully some day I will shoot with one. I love that camera. It lets you light way more creatively since the highlights just go on forever and the noise texture is really beautiful and organic. It puts the Epic to shame unless you don't mind clippy highlights and need a high res, digital, plastic, inorganic look, in which case the Epic is genuinely better. But in less-than-super-genius hands and without a totally insane post budget the Alexa is a much better camera (and I'd argue most super-geniuses still shoot better with it than with the Epic).