My apologies for using words like 'nearly identical' in a thread that asked a simple question, but for which someone had great heaps of information to disseminate. I am bowing out of this "conversation" to answer the original question
again to the OP since the point I was attempting to make has been co-opted, derailed and otherwise lost.
Mark, suffice it to say, I attempted to test this yesterday with a D600 and D7100. Given that I do not have 2 fixed length lenses that would be considered of equivalent focal length with the different body types I attempted to approximate it with a zoom lens. Alas, I was not able to exactly match the two photos even with the lens mounted to a tripod and the bodies swapped - either the lens moved slightly when swapping bodies or I was never able to dial in a perfect "105mm" on FX for the zoom's "70mm" on the DX. Suffice it to say, discounting the minor changes in orientation, I would be hard pressed to tell you which image came from which camera without looking at EXIF. File sizes were about 1% larger on the D600 (32.1MB vs. 32.3MB). No noticeable difference in sharpness, clarity or lighting. If you'd like to see the "almost matching" shots, PM me and I'll put them together for you (and only you). I will attempt to complete this comparison in the near future if only to be able to post files that will allow those who want to make the comparison to do so without having to wade through the deep waters here. I figure if they can play with files for themselves and form their own opinions then what better method, right?
So, ignoring the lessons on optics and answering your original question in the spirit in which I believe it was asked, I find
nothing that would lead me to point you away from the D7100 for the uses you describe. You have a D610, and obviously appreciate the impact glass can have. So, if you're putting good glass on the front of it, landscapes should be of no issue for you. Yes, you
might be able to pull some more light out of shadowy areas in difficult lighting with the D610, but at the same time I believe the absence of the OLPF on the D7100
may actually provide you with a touch more clarity on some shots as well, so there's a
potential trade-off.
I must add that as an owner of both, were you to press me to cut down to one camera then I would choose the FX. While Wayne's dissertation has made it difficult to discuss the topic as I believe you intended, the details behind it point to the fact that the D600/610's sensor has distinct advantages in terms of its ability to resolve light. These
may not be perceptible in all situations, and will likely not come into play in the large majority of your photos, but I suspect that there will be situations in which the D610 will give you something (more) usable that what you'll get out of the D7100. I'd put a percentage guess to how often, but I suspect it would be questioned.
