Sliding and Downhill are two very different things... Dont try to learn and slide on a Dh and dont try to Dh on a slide board.
Very simple, you need two boards

Landyachtz/rayne/kebbek all make good dh boards, take a look on their web sites and go to the wiki skate to learn about how different boards perform, then find something that works for you
http://wikiskate.com/wiki/index.php?title=Main_PageMost Dh boards and some silde boards are talked about on the wiki.