If it does make a difference SEO-wise I think the impact is so small that it can be ignored. I do think it is a good idea to force your traffic on 1 of the 2. In my case it works both with and without www, but all incoming traffic is forced to the non-www URL to prevent search engines from seeing it as 2 sites instead of 1. The reason I went without www is because it is, well, shorter :p. People are lazy, the closer at hand something is the better.
Edit: wtf, apparently my www. url went on a vacation >_>
Edit 2: ignore people who say use www. without providing valid arguments why, either 1 of the 2 is good, 2 at the same time is bad.