There’s a couple reasons a sports photographer (which I’d assume the adorama guy was) to choose the D500 over the 750. One is buffer capacity (basically the super fast memory built into the camera, where it stores the data off the sensor momentarily before writing it to the much slower SD card) is larger on the D500. That’s why it can shoot 10fps vs the 6.5 in the 750. Is that important for you? I’m not sure. only you an really answer that. However, I’ve shot with the 7100 and 750, and have never really found it necessary to shoot at 10fps. I am not a sports photographer, and don’t need to freeze action of someone hitting a baseball or making a tackle or whatever. I have shot quite a bit of kids running around like at the ATI conferences, some guys at Nantahala river in kayaks who were doing trick and stuff like that. For kids, 6fps has always been plenty. For the kayakers, I did need it a bit faster. However, I was outside in a well lit area, and figured I wouldn’t need to do much post processing on the images. So I switched the camera to shoot in jpeg, which are smaller files and can shoot faster (since the buffer is big enough to hold many more jpg than raw). That was plenty fast enough for that situation.
The other reason a lot of the same people who want high frame rates tend to like DX sensors is the crop factor. It means they can get more “reach” out of the same lens. The adorama guy’s 200mm lens is the equivalent to a 300mm lens on a full frame camera. So, it’s better suited for shooting fast, far away subjects like at a football game.
You *can* shoot at 6400 iso on a crop sensor. And you can make it look ok with some post processing. Personally, I never really went over 3200 most times on my 7100, because I wasn’t happy with the image quality. DX sensors just get grainy at high iso, no getting around it. Because the light receptors on the sensor are smaller than a full frame, the only way to make up for it when shooting in the dark is to boost the iso, essentially taking whatever light is hitting the sensor and amplifying it. If you’ve ever turned the speakers in your car up very loud, you can hear the sound gets distorted. The same happens to the signal coming from the sensor in your camera. If you try to amplify it too much, it gets distorted, and you’ll start to see odd colors and grain show up in your image. The only way to avoid this is physically make the sensor larger – full frame (or medium format, or even larger!) – to capture more light in the first place!
That leads me to my reasons for buying, and experiences with the 750 vs the 7100. I shoot in a lot of dark places. Much of the time it’s stationary stuff like stars and trees, so fps has never been a priority for me. however, I’ve shot more than my share of speakers at conferences, kids running around at VBS etc. in dimly lit or dark rooms to say that the 6.5fps of those two cameras has pretty much always been adequate. plus, for me, the benefits of the full frame in the D750 MORE than make up for the lower framerate. Is your situation the same? I don’t know, but that should give you something to think about.
the tl;dr version of all this:
Crop sensors are good for fast frame rates and gaining some extra reach from your lenses
Full frame are better in low light.
Imo, the D750 is just about the perfect balance of camera for just about every situation. It’s not the best at one or the other. Dedicated sports pros might want the D500, D4s, etc. Dedicated landscape pros might be better served by the D810. But, for me who shoots a little of everything, but likes landscapes and shoot often in low light, the D750 is hard to beat.
You might check this article out, it probably will answer other questions you might have. https://photographylife.com/reviews/nikon-d750/7