Re: E-5 better in low light than D7000? Yes, maybe.
I have found that 12MP is enough for all I do. Another side of this is that there are few lenses that can resolve more than that on a crop camera. When looking at the low ISO samples from dpr, the 12MP E-3 shows considerably more detail than the 18MP 7D. That is probably a combination of lens and filtering, but there isn't really much point in an 18MP camera if the lenses can't render that much.
This is of course what Olympus has been pondering all the time, without getting much response, but it seems to be true: 12MP and top notch lenses will render more detail than 18MP and lenses that were designed for full frame.
When it comes to DOF, it's very much a question of taste. I mostly shoot portraits at f/2.8-4.0 on full frame. That means that the Sigma 50mm f/1.4 will do the job very nicely when used on a 4/3 camera.