The main things I like about the EVF are
- Massive FF sized view
- Focus peaking in manual focus
- Choice of overlaid info (histogram is useful)
- Shows you a very good approximation of what the shot will look like (no more under/over exposing by accident). (You can switch this off if you want)
- Gains up in very low light to be able to still see.
What isn't quite perfect
- Slight drain on battery life (but spare batteries are cheap!)
- If you are being picky, very high contrast scenes aren't rendered 100%
- If the sun is massively bright and overhead, you have to ensure a good seal with the eye to the eye cup.
- Can be a fraction laggier then an OVF at high fps
As ManCuBus says, the EVF is love/hate, and I think the acid test is just playing around with the different cameras, you soon start gravitating towards one, and it's not like you can make a bad decision!
I'd go Nikon as once you start buying lenses you're stuck with a brand and I think the higher end range of cameras and lenses are better on Nikon.. I'm sure either camera will suit your needs.
I don't disagree that at the top Pro level, Nikon are that step ahead.. But that's only top end pro..
Sony have the whole Minolta back catalogue to fall back on, and have their own 'G' range which are totally on par with any other brand.. They have a wealth of mid-range lenses that are superb value for money (look no further then the kit 16-50 f/2.8 SSM), and all lenses get IS as standard.. They do have some 'holes' in their lens line up, but all brands have gaps in their lens line-ups. And the new A99 takes you easily up to the D600/D800 range, Sony just don't do any higher, so D4/1DX is where their gap is..
I don't totally subscribe to the idea you are locked in, I've switch brands twice, I just sell/buy a few lenses secondhand which is usually about cost neutral, it's only if there is a new good lens do I sometimes re-buy, but that's only because I want the newest version of that lens..