VF2 and VF3 viewfinders

I'm currently weighing up whether to get a NEX 6 or an Olympus EPL5 as an "out and about" camera and have a viewfinder question. I appreciate that if I went for an OM-D EM5 I'd have a viewfinder built in but it's considerably more expensive, is less compact and doesn't feel right in my hands.

I've tried lots of mirrorless cameras out in camera shops and I found some of the EVFs quite "shimmery" to look at (on the Panasonic G3 for example) - I'm not sure why or what specification might tell me whether a particular EVF will do this. I find the viewfinders on the EM5 and the Nex 6 and 7 fine. What I can't try out in my local shops are the optional VF2 and VF3 viewfinders. Can anyone give me a comparison to the built-in viewfinders on various models so I might get an idea of what they're like?