My 2 cents
1) If you have a good amount of backlinks on both keep them booth. A good
PR on both will do fine. If your selling something you can redirect the non-www to a productpage and www to the main one. This way you build a strong innerpage.
2) If you just started just use www. like kaykays says. Most people use that, and just redirect 301 the non-www to www.