Man, it's so hard to recommend one because they are both great cameras, and there are things about each that I wish were on the other camera. For example, the D800 is great for wildlife and sports because you have a better usable reach since shooting in DX mode you still get 16MP's. But the D610 shoots at 6 fps while the D800 maxes out at 4 fps in RAW mode, which is a PITA when you want to shoot a 7 or 9 frame HDR series and are handholding for 2 seconds. I prefer the low light/high ISO performance on the D600/610, but love the resolution on the D800 for things like macro photography. I have no issues with IQ on either, though I find the D600/610 to be a little smoother.
Because of the reach and resolution, and the sturdier construction, if I had to go with one and only one body I'd go with the D800, though I'd opt for the 'e' version if I had to do it all over again.
I'd go to the NikonUSA site and use the compare tool with the D800e, D610 and the D300s, and run down the range of features and see where the similarities are to your D300 where that is important, and which of the others win out in the other categories. Coming from a D300 I think you'll likely be happier with the feel of the D800e, but if what comes out of the camera is as/more important then it's down to features.