Yusuf, as usual, a valid answer to your question is... it depends. depends on your budget and depends on your main focus of photography. If your prime use is for wildlife, then it is a no-brainer to go for the D7100, but if it simply general photography, with some wildlife thrown in for good measure, then the D750 is a better option. Either way, you are going to have to upgrade your equipment in the lens department in order to get the best out of any body you decide on. Your 18-200 will simply not do justice to either body. Both have 24MP sensors, but given that the D7100 is a crop-body, any bad technique and/or camera movement will be compounded on this body, and low-light photography will simply not be as good. All indications are that the D750 will have a better low-light capability, given the size of its pixels compared to the pixel size on the D7100. Insofar as reach is concerned, the lenses you will require for the D750 will have to be longer than the lenses required for the D7100 if you wanted to get the same field of view, but you do have the benefit of a 24MP sensor, which should allow for a reasonable amount of cropping to take place on images from either camera.
Personally, I use a D7000 and a host of different lenses, but few are at professional level. If I were in your position, I would first upgrade my lenses to a professional-level quality, and use the benefits of the D90 - you will see a major difference to the quality of your images - and I would do this BEFORE you spend money on a new camera body that simply cannot be exploited to its full benefit with the lens you currently have. I would suggest that you DO NOT throw good money after bad, by upgrading your camera, without upgrading your lenses.... and clearly, the lenses you purchase will depend on its intended uses....
"Take nothing but memories, leave nothing but footprints"
Learn to use your equipment
and spend your time taking,
rather than processing images.