Well right now there's only one game that supports DX10.1 and that's Assassin's Creed. It actually gives the HD3850 a pretty big boost while playing it too. Although there's not enough games right now to really justify going for it for just that reason, though hopefully, more games will start to support it.
The 9600gso will never support DX10.1, it's a DX10 card and that will not change unless they decided to release a D10.1 version of it.
As far as which card to go with, I'd go with the HD3850. It performs a tad bit better than the 9600gso, which is just a rebadge of the 8800GS. With AA enabled the 8800GS comes out a tad bit ahead on some benchmarks.
http://www.fudzilla.com/index.php?option=com_content&task=view&id=5354&Itemid=40&limit=1&limitstart=1
If you ever plan on using an HDTV with your computer, I'd go with the HD 3850 over the 9600GSO any day of the week. Otherwise, look at the reviews, see which games you play, and which card performs better at those games.