You can always upgrade gear. Any lens or camera body has an alternative that will have more features for additional cost. That doesn't mean you really need it.
I have a D800E and bought a D600 when it was released. I've used them side by side as well as used just the D600. There are differences. But for most photography, the D600 is a great camera and a substantial upgrade. I don't think you need the D800/E to be happy unless there are specific features you are missing.
The ergonomics are close, but there is a difference in layout of controls on the body. For the most part the issues are not significant.
There are a few places where the D600 falls short. For example, it only allows a 3 frame multiple exposure which I find silly. But unless you really need this feature, I don't think its a show stopper.
If you have been content with a D90, the D600 is a good choice. If there was a deficiency with the D90 that prompted your upgrade, you could look closer at features of each.
The reason to select the D800 is you truly need advanced performance. If you simply need a good camera that produces great images, the D600 is fine. Don't second guess yourself needlessly. And don't underestimate the impact on lens selection - with the resolution of the D800E you become much more demanding on softness and lens issues.