sharkman53: you haven't mentioned what or how you shoot. That will make a big difference in whether or not the D7100 would work for you.
That said, I was in a pretty similar position a few months ago. I decided to bet on the D7100, and for my particular needs, that bet paid off. I wrote up my thoughts on my blog, but will rehash the main points here
— The low light performance is at least two stops better than the D300. This may not matter so much for you, given that you already have a D3s. That said, if you want your backup body to work in the dark, the difference here is (*cough*) like night and day. This is 6400ISO with very light noise reduction: http://web.mit.edu/~xsdg/www/pblog/2013-apr/jupiter/710_3417.jpg
I was stunned when I pulled that image off of the camera. The D300 simply can't compete with that kind of resolution (and I say this having printed a couple 24x36es and a bunch of 16x24s from D300 files). If I had shot those same images with a D7100, the difference in the final images would have been easily noticeable.
— The buffer is really small. It's usually okay, but there have been times (during event coverage, not even sports) when I've been taking images of a rapidly-developing scene, and had to wait for the buffer to clear. I only ever use single-release (aka no continuous), so that's not the issue.
— I really dislike the physical attributes of the D7100 body. It's too small, it's difficult to grip, and many of the buttons aren't in the right spots (whereas those same buttons were perfectly-positioned with the D300).
— The shutter button is mushy. With the D300 shutter mechanism, the "half-press" resistance is different than the "full-press" resistance, and there's a hard boundary between the two. So it's trivial to half-press quickly and know that you're on the cusp of firing the shutter without actually firing it. That hard cutoff doesn't exist on the D7100 shutter mechanism, so it's more difficult to know where that boundary is and to find it quickly.