Sorry, not intended to be a lecture, just was thinking that the cameras can all take pictures. It's up to the photographer as to how they come out. Each has advantage and compromise in its use.

A DX sensor is about 2/3 the physical size on an FX sensor.
The FX sensor will typically have better low-light performance, due to larger photosites. 7100 vx D600/610.

The sensors have different "bokeh" characteristics for the same focal length equivalent lens and aperture.
A DX sensor will be perceived to have more "reach" with a given lens than the FX sensor. This is in terms of field of view.
Example: the 18-200 DX lens on a DX sensor is the same range as the 28-300 FX lens on an FX sensor.

Each has advantages and compromises, that affect the final image.

Learn to work with what you have.

The buffer size of the D7100 probably didn't change from the d7000, so of course burst length performance suffered. There are also some things that lower the buffer available for images that you can turn off.
Long exposure noise reduction, Hi ISO noise reduction and lens corrections come to mind.

You can also lower the picture size or quality settings to gain some buffer time.

Sad that Canon's upgrade to the SX50 is considered a downgrade by users.

Increased zoom range on both ends.
slightly higher resolution.
better EVF.
Wi-Fi built-in

But still not perceived to be as good as.

The A-12 was developed with the black budget of the CIA. It was a single seater, thus very busy for the pilot.
The YF-12 was a proposed interceptor version.
The SR-71 replaced some of the equipment bay area with a second seat.
The general configuration of all of them was similar.
And it was designed with slide rules.

In Corpus Cristi, the oldest WW2 aircraft carrier has been turned into a Museum. Oldest by virtue of the keel laying date, the Lexington, CV-16.
In Mobile, AL, there is the Battleship USS Alabama, BB-60, one generation before the big four.
These qualify as landscape, and a break from wildlife.

Have fun!

Not quite a D7100/D810 comparison, but I had a pair of Canon's out on Saturday in 10-15 degree F. weather.

The SX60, with it's larger battery kept going as long as I needed it to. The SX280, with a much smaller battery showed empty almost immediately. Replaced the battery with one from an inside pocket that was fully charged the day before, and again, showed empty almost immediately.

After the SX280 warmed up a bit, the batteries started to show a normal charge again, without recharging.

Had the D600 out the week before, but it wasn't quite so cold, no issues on that trip.

Same situation: Big fingers, small area between grip and lens.
When my fingers get in the way of the zoom mechanism, I get that message. Most times, I have the hood mounted reversed, and it hits my fingers.

Solution: keep the travel path of the lens clear.

Nikon started not including a hood and case with the 18-140 lens last year. I suspect that other new or redesigned lens will also stop having these items, especially the less expensive ones.

Example: the newly announced 55-200 kit lens does not have them "in the box", but the 300 F4 will.

THe D7100 is a bit bigger and if you add the battery grip...

The jpeg editors I've used have always had a quality setting. This attempts to limit the loss of quality as the file is saved. Using a high quality setting will result in less degradation of the picture. Using a lower setting will generate a smaller file, but with loss of details. Try it out in a controlled manner, and see what happens.

Cables are nice, but if you already have one, or if you have an sd card reader, that works also.

Definitely 55 to 57. With what can be seen of the tailfins, I would say it is a 57.

There is a manual focus setting that will allow you to narrow the autofocus zone, but you can't completely manual focus since the push buttons don't allow it. Page 79 in the download manual from Canon shows what to do.
Press MF / macro on the four way switch on the back, arrow left or right to choose MF, up or down on the switch to set focus. Won't work in auto mode, so change to something else. Zoom focus can help.

Maybe with practice could do it handheld.

A good outcome for both of you. The d7100 pairs well with the 18-140mm. I kept that combo also.