Have you looked at the D300 forum? There has been a lot of wailing and gnashing of teeth over there about the lack of a "D400", whether the D7100 is an adequate successor to the D300, and what features are missing in the D7100 that invalidate it as a D300 successor.
I came to the D7100 recently from the D90, which was a smooth transition and (in my humble opinion) a significant step forward that gives me (and maybe only me) all that I would have asked for in the mythical D400. However, some (but not all) dedicated D300/D300s shooters who do not feel the same way about the D7100 as a way forward for them.
I think it all boils down to features. What do you want in a crop factor body that you do not have in your D300? What aspects of the D300 MUST you find replicated in a crop factor body with a next-generation sensor? Only you can decide whether the advances in the D7100 are sufficient to overcome the D300 crowd's reservations -- most of which, in my rather naive view, seem to revolve around the fact that there are "pro" features in the build and layout of the D300 body that are not found on the D7100.
I think the D7100 is a pretty amazing camera, but I did not come to it from a D300/D300s perspective so I may not recognize what all the angst of D300 shooters is about.
After that long-winded preamble, here's my advice: get your hands on a D7100 and see whether, for your intended pattern of use, the deficiencies perceived by D300 users are important to you. If they are, hold out until fall to see if Nikon actually turns the myth of the D400 into reality.
I have shot with the D300s and D7000. Not exactly what you are considering but I think close enough to comment.
The D7th is an amazing camera and I am sure the D7100 is as well. However, they are very different cameras compared to the D300s. The body of a D300 resembles that of D800 in terms of size, buttons, etc. The feel of the cameras is totally different. Although the menus are very similar, the external buttons and size make it totally different shooting experience.
D90 to D7000 to D7100 is a natural progression and an upgrade. Similar size, buttons, etc....D300 to D7100 is basically changing cameras. You will not blend right in. If you want to make a total switch, then I would consider it. If you are set with the feel of the D300, then wait for the D400?? or go to a D800.
The D7100 does not have a 10 pin Motor Terminal Release port and has a slower FPS rate than the D300/D300s. It also has a slightly less robust build quality. These are some of the primary reasons that many are still hoping for a D400.
As others have suggested, there are differences that appeal to different priorities. If frame speed is the main consideration, there is a minor difference in favor of the D300, but that assumes the lower quality 12 bit compressed file. At 14 bit, which gives more flexibility in post processing, the D7100 is considerably faster. In all image quality parameters, the D7100 is better. The controls are different, with the D7100 having a few less external buttons but more in line with newer higher end cameras from Nikon, such as the AF mode selection style is the same as D4, D800 and D600. What are you shooting with DX and with the D3s? What are your priorities and eventual display method? Small files for the web won't make much difference, shoot in lower light, need post processing flexibility, printing high res or large will be dramatically better with the D7100. Stan St Petersburg Russia
I went from a D200 to a D7000, and never looked back. The D7100, that I now own, is built like a light, solid brick. The AF is very fast, the image quality is super.
I'm not the biggest fan of SD cards, but I LOVE the 2-card, backup option (plus others with he 2 cards).
Heavier does not always constitute better, and unless the larger buffer is really critical, I can't imagine not going to the D7100. It is the best DX available, from anyone, and should a D400 ever arrive (at what will be at least $1800.) you can always sell and shift...
The D7100 may not be in the exact mold as the D300s, but it decidedly replaces it.
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.
I loved my D300 -never missed a beat ( after sorting forward focus early on) and now I love the 7100 more. It takes a little more work to understand at first but now its great. Just done a trip away ti US ( live in Sydney) and all images great. Not really interested in Video but tried that and it was amazing.
it also loves great lens makes them work well.
Not sure what to do with D300 as trade in was so low I guess just keep it as a spare.
confident you will be very pleased with results - only shoot RAW Geoff