Based on the lens options you listed, all of them will have the same low light capability - meaning they have at the same max aperture equivalent for their sensor.
Difference will be their sensor size advantages/disadvantages of low noise/camera size etc etc..
Talking about Sony vs other brands, not putting the others down but the EVF advantage is huge. Yes, live view is close, but you lose the focusing speed. See Gary Fong's video which explains it very well.
Yes, I use the Sony, primarily for size and also with my long sight problem - live view is a real pain - EVF takes care of it while retaining all the live view features.
Lets ignore brands, prices, the physical size and weight of camera and lens. To make it tough, you can only choose 1 of the following. Option 1 - full frame + f4 Option 2 - aps-c + f2.8 Option 3 - micro4/3 + f1.8
It's a great camera this Sony but how important is AF to you? If you intend to keep your Nikon lens to use with it, you might be disappointed as it is the most sluggish compared even to Canon using the metabone adapter. I intend to keep my Nikon FX lens at least till I can afford to upgrade them to the Zeiss...etc when i get a mirrorless but even then, i am not looking to change my D4 for the present generation. But I can see my future in another few years down the road to change to a mirror-less but it will not be get a lighter gear. I am fine with heavyweight for stability.
But the technology that is going into the cameras like the Sony... Silly Canon and Nikon is too slow to embrace which i think if they don't will be their lost.
I take the Full frame + F4 like my 70-200mm any day. It is awesome.
PS: The battery life is also a bother to me to have to buy a lot more to charge and bring around. Hope that will improve when I get down to buying the next 2 generation if Nikon do drop the ball.
I just sold all my Nikon FF gear as well, but I switched to M43. The specs and reviews for Sony's newest cameras and lenses all looked amazing, but it didn't seem too different from my Nikon stuff, and the weight and size were the biggest issues for me.
I'm guessing the size and weight were not the main issues for you then; do you mind sharing why you switched? Curious ;p
If i am limited to only a F4 lens on full frame, i will buy a m4/3 with F1.8 lens.
I bought into full frame systems because I want to achieve the shallow DOF and effects that is much harder to achieve with a m4/3 setup. Smaller, lighter, faster shutter speeds (use lower ISOs), whats not to like? Bokeh will be close or equivalent to the F4 lenses on full frame.