Thanks to everyone for their input. I am leaning towards keeping the D750 but am going to spend some time over the weekend, and maybe over Christmas, before deciding.
The D750 will be better if you push the ISO but for pictures taken with 100 or 400 ISO you won't notice much of a difference....I used the D750 for a year along with a D7000 and people can't tell what camera body I used while using the same glass on both.....When you upgrade your lens then you will see a difference in your images....The D7100 is a great camera.
This is one of the concerns I was alluding to in my initial post but maybe didn’t come across so well. If I am taking photos that are in the sweet spot of the D7100 capabilities is the D750 “wasted” on me? Ignoring the additional cost of glass I would need to buy for the D750 to benefit from the full frame body I could add a couple of decent DX lenses for the D7100 for the additional value of the D750 body. I know I said that I wasn’t as concerned about the cost per se but it should always be a consideration unless you literally have money to burn as there is always something else you can spend it on. i.e. if I got a 35mm prime for the D7100 my equivalent focal length would be similar to the D750 with 50mm and give a similar field of view for indoor shooting. I appreciate the D750 has the bigger sensor but in general shooting is it overkill?
I completely get that there are situations when people prefer either FX or DX but given I don’t know where the craft will take me (“general” use and family work is vague I appreciate) does the D7100 provide me with just as good a middle ground for a lot less money?
Not prying, but have you considered keeping 2 of the 3 bodies? There are instances in shooting wildlife/sports where having 2 different lenses ready to shoot can be a blessing. Great introduction thread btw.
My wife only wants one as she thinks it is a waste of money to have “two big cameras”. She has a point and shoot and we both have phones so she would see keeping both as gadget-lust overkill