I would say that part of the question is whether or not you'll keep the D700, or even if you are considering it depending on your happiness with the crop-sensor camera. It sounds like the new sensor (assuming it's the same as the K-5's) really is living up to the hype of offering D700 ISO performance in an APS sensor - and higher resolution, to boot. At the very least, it's awfully darn good and much better than the D300s - and probably much better DR, also which is something you may miss if you went from the D700 to D300s.
In terms of video, from what I understand, all Nikons prior to the D7000 recorded audio at 11kHz, vs the more standard 44kHz that all other DSLRs use. So you might notice a difference in the audio. 1080 is nicer than 720 but I'd rather have 720 at 30fps than 1080 at 24fps. Sorry, film snobs (and I am a lover of classic film, as you can tell by my username!), but 24fps doesn't make much sense for home videos in 2010. You get more jittery movement and there aren't many display devices capable of displaying 24fps correctly. (This is a nice thing about 24fps Blurays on 120kHz TVs, but I doubt they'll play back your DSLR video this way.) B-frames and such are fine for lab-room measurbating but at the end of the day, it's all about how it looks when you play it back, and that is going to be something you probably won't know until you play with it yourself.
One other thing about video - the D7000 is the only DSLR to offer autofocus in video mode. Of course, I'm not how effective it will be, but it's there. The K-5's contrast-detect AF in liveview mode is astonishingly fast so the current gen should be capable of doing it pretty well (I have no idea how the D7000 is in this regard.) This could be the difference between it being an occasional toy and something you'd use more regularly. I am pretty satisfied with the video I get from my camera but manual focusing while shooting is a bit of a pain, and it doesn't help that you're likely to move the lens barrel a little while doing it, giving it a little wobble.
I doubt there's any reason to be concerned about the tripod mount. How often do you hear about it ripping off the camera? Plenty of people use R-Straps on plastic-bodies DSLRs with no issues.
Were I in your shoes, the only thing pushing me towards the D300s might be improved weathersealing - I'm not sure that the D300s has "full" weathersealing but I don't think the D7000 does. The AF may be faster in the D7000 too, being a newer model, so that's not a guaranteed plus for the D300s, either. At the end of the day - the cutting-edge 16mp sensor is too important a feature to overlook compared to the old 12mp sensor which is, compared to newer sensors, looking a little bit long in the tooth. If you did need the D300s build/AF, I'd wait until the replacement with the 16mp sensor comes along.