I'm not sure but I think the Tiagra RD will work for ten speeds, though it would be a shame if you got it, upgraded to ten speeds, and then it wasn't compatible.
105 will be a bit better and lighter, but its up to you if you think it's worth it. From my experience, Tiagra works very well. However 105 (especially 5700) is suppose to work nearly as well as Ultegra and Dura Ace.