The folks over at Phoronix did a comparison of 3 of the major Open Source operating systems and concluded that each had different strengths, but simplifying things Linux bested OpenSolaris which bested FreeBSD. It's not all that clear though since they tested a final version of Ubuntu (Intrepid Ibex) against a Release Candidate of OpenSolaris and a Beta version of FreeBSD.
They conclude with "If simply counting which operating system was in first place most frequently, it would be Ubuntu. Ubuntu 8.10 x86_64 was in first place eight times, OpenSolaris 2008.11 RC2 was in first place seven times, and FreeBSD 7.1 Beta 2 AMD64 was in first just three tests. Depending upon your system usage, one operating system may appear more favorable, like OpenSolaris with the greater disk performance."
As usual there's a good treatment of the results over at Slashdot. I'm looking forward to doing an install of FreeBSD and getting some first-hand experience to see which is faster from a Desktop perspective