If the D300s does what you need, buy it. The D90 does almost everything I need, but it won't meter AIS lenses, so the only choices I have are D3, D3x, D700, D300 and D300s. Since I'm other-wise very happy with my D90, I'll opt for the D300s. Gives me the metering I'm seeking. I don't need more Mega-pixels - 12 is plenty for my needs. (In fact, the D40 has produced some of the best photo's I've seen in recent years - a photographer friend loves it, carries it almost everywhere and shares what he gets. Help's he's a private pilot and shoots extensively from the air over San Francisco.)
I also don't need full frame, so for me the smaller size and lower cost of the D300 makes it the most desirable. Getting the D300s, just gives me a couple of improvements that I might find valuable (multiple memory cards).