Well I have both the D7100 and a D750. If you're primarily a wildlife sorta guy I have to wonder if you're going to miss the crop factor you're getting with your big Tam' on the D7100. If you intend to keep both bodies I think think you're going to find you like the DX body better for that particular type of shooting. That being said, I find the D750 is a dream to shoot with, generally speaking, and if "master-at-none", it's definitely a superb, "Jack of all trades".
How do the D7100 and D750 differ in handling? Well the D750 is a little thinner in the grip and the top LCD is configured a little differently but the button layout is pretty much the same, both top and rear, on both bodies so I find I can switch between bodies pretty much effortlessly. Menu configuration is pretty much the same as well though the D750 has some different options of course.
I'm not sure what you mean by "software options".
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