Personally, I would be surprised if any theoretical differences were appreciable in practice. There are many other factors affecting the speed of a game server that probably drown out OS differences.
Go for the OS that you feel most comforatble with and that offers the features that you require. For instance, on any sort of server I would go for something that supports ZFS, which rules Linux out. But what is important to me may not list highly on your priorities. Performance (IMO) is almost always best dealt with at the hardware level.
Well said! Funny you mention ZFS, Solaris 11 does indeed use ZFS by default which I definitely like. Well I guess I'll stick with Solaris 11 then. If anything Solaris should be faster than Linux with Java because you know Solaris and Java are under the same roof but who knows.