Strangely I had my heart set on the D7100 before I bought the A77, but after trying out the D7100 in store it wasn't for me due to the slow AF in live view, and lack of AF in video as well as not being able to use the viewfinder in video. But the biggest thing was the ergonomics, the Sony wins hands down IMO. These things aren't light and so ergonomics are important. According to reviews the Nikon is supposed to have arguably better IQ, and less noise. Plus Nikon supposedly do the best glass, although the Carl Zeiss and sony G range are far from shabby.
As for the electronic viewfinder, it's a revelation in the A77 and A99, they've really nailed it. Some say it's not as good for fast panning but I've had no issues. The only thing is there's a slight 'break' in continuous shooting ie the screen goes blank for a split second while capturing the shot, so if you're panning you might lose the subject. Again I don't find that an issue, but it's certainly a disadvantage over the traditional viewfinder.
The advantages of the EVF are the ability to see through it at low light, but the main one for me is seeing setting changes in the viewfinder. For example, if you change white balance, exposure etc you see what these changes look like before you take the shot. Also you can choose to have the histogram and other nice features in the viewfinder too.
Peaking is a nice feature of the A77 (not sure if the nikon has it?), and the adjustable screen is great, especially when using a tripod.
Both cameras are superb though and Im sure you'll be happy with whichever you choose.