I've said this a bunch of times about the D750 - it's a step up in IQ, particularly when you're dealing with high ISO and the need to pull dynamic range out of a single photo. But, with that said, under most normal shooting conditions it is not an appreciable jump to warrant moving from a D600/610 to the D750, but it's appreciable enough that if budget isn't a concern that I would definitely recommend buying the D750 over the D600/610 (though it would seem that the D610's are now incredibly priced and more than worth the going rate).
I wholeheartedly disagree with the assertion that the "D810 should outperform it. Most pixels usually wins." More pixels can have the opposite impact on IQ depending on the situation, and if you're shooting landscapes, particularly at small apertures for maximum depth of field, small pixels and small apertures can cause a loss of sharpness. I've all but abandoned my D800 in favor of the D750 and D610, and only still have it because it's not worth me taking the nearly 50% hit on the sale, so I'll keep it for those rare moments where studio resolution is an absolute requirement. But that said, the lack of anti-aliasing filter will also give a boost to sharpness, so it does win when used to take advantage of that. The file sizes are a consideration with your computer, both in terms of processing and storage, so be ready to buy more disk space, because it fills up fast, particularly if you're going into Photoshop and saving Tiff or PSD files.