"The D7100 may have a more modern sensor, but as far as I am concerned it is a step back in every other respect."
I disagree. The D7100 is an improvement over the D200 in MANY ways. Same can be said for the D7100 vs the D300s. Lets put it this way: There are a few (very few) things that a D300s can do that a D7100 can't (namely shoot at 8FPS with a grip, and offer an AF-On button, and a 10-pin connector, and the buffer is smaller if you want to take 10-15 continuous full-res frames in Raw). From what I've read on the sports forum, good photographers don't do that very often? These may, or may not, have a practical impact on your shooting.
By contrast. there are many things a D7100 can do that a D300s can't (like shoot at ISO 6400 with the same noise that a D300s has at ISO 1600, crop the bejesus out of an image, and allow for much better dynamic range). Plus, I think the center sensor can AF at f8, which can be useful with teleconverters. The AF system is newer. The metering algorithyms are newer. I don't know anybody who can't stand to benefit from those improvements.
That said, you're still using a D200, so a D300s would be an order of magnitude jump for you. I have a D90, which has the D300 sensor in it. It is vastly inferior to my D7000, which is a generation behind the D7100.
BTW, I also have a D50. It was my first DSLR in 2005. Don't trade yours. It's not worth anything (maybe $50-$75). It's small, light, and easy to use. I still use mine for snapshots and architectural documentation for my business, where 6MP files that are never going to be printed on anything bigger than 11x14, are more than enough.