- Like what you've got: stick with it. There's nothing particularly wrong with the D7100, though it does have a limited buffer.
- Dislike what you've got: You have three basic choices: (1) move up to the DX Pro body upgrade (e.g. D500); (2) get a D7200; or (3) move up to FX.
Which should you do? Here's the simple guide:
- Choose #1 if the buffer is really your primary problem and you’ve got a lot of disposable income ;~). (You may think this is a snide remark, but it has weight behind it: you’re giving up a built-in flash, for example, so what you gain had better be very specific to your needs.)
- Choose #2 if you're happy with what you've got in terms of camera and with DX in general but want to upgrade simply to update your equipment and get better body quality and performance.
- Choose #3 (move up to FX, most likely the D610 or D750) if you're now serious enough that only the best possible image quality will do for your next camera. Be prepared to have to upgrade your lens set, too, though, especially at the wide angle end.