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Just a note on overall size...the native lenses for m4/3 are significantly smaller than the native lenses for NEX, and adapted lenses will tend to be larger as well, since you'll be using longer focal lengths for the same FOV.
The sensor size on the NEX can be somewhat of an advantage with adapted lenses if you prefer shallower depth of field. However, in image quality, there's very, very little difference between the two sensors aside from a slight resolution edge for the NEX. High ISO capability is identical.
It depends on your needs, in terms of lenses. For me, upcoming Sony 35/1.8 OSS is actually smaller than the Leica 25/2, and the NEX-7 is smaller than the OM-D. The Sony 50/1.8 OSS is quite a bit larger than the m4/3 45/1.8, but the NEX-7 is smaller than the OM-D. Wide angle-wise, the new Sony 10-18 is smaller than the Panny 7-14, I believe, although I use the Sigma 19, and I think it may be larger than the equivalent m4/3 lens. Not sure.
In terms of adapted lenses, the tough thing is that, as the lenses get wider, they tend to get slower, and, if we're talking about M lenses, they tend to get larger, too, unless you're talking about teles. The CV 35/1.4 that the OP mentioned is a pretty fast 52mm equiv. on NEX, and it's harder to find an equivalently fast 25mm manual lens for m4/3, unless, of course, you go with the much, much larger 25/.95.
The big issue with the OM-D for me is the height from the "prism." Unless you maybe use pancakes, the lens length dimension of these cameras tends to be the longest dimension, anyways, so I'd rather have a little extra length in the lens dimension, rather than adding height to the shortest dimension, in terms of packability. Even if the OM-D + lenses is technically as small or smaller in volume than my NEX-7 with somewhat equivalent lenses, I couldn't put the OM-D in some of the slim bags that I like to use. I had the same problem with the NEX-5N with its EVF attached, which is partly why, although I like the 5N EVF's tilt, I traded it for the NEX-7.
I agree about IQ, for the most part, in that there seems to be little between the two in terms of low light performance, except maybe for the very high ISO. Granted, at least according to DXO, you do get a tangible DR difference at base ISO. I'm sure either of these sensors would make most happy.
edit* To add to the above, I should say that I think the OM-D looks really good, and, while the prism height is an issue for me, it does balance out the size of the lenses nicely. The NEX-7, being so short, really accentuates the lens size, in a bad way.