I wrote a simple program to go through the first X hostnames in the alexa top million list, and query the sitedossier.com database to compare the count of inbound links pointing to www.example.com versus example.com. Here are the results showing the winners.
(Note that if either version of the hostname has less than 10 referers the site is excluded.)
www.example.com: 85 (86.7%)
www.example.com: 758 (86.3%)
www.example.com: 4,989 (84.6%)
www.example.com: 20,371 (79.8%)
There's a clear winner in all instances!