The SciMark results were mixed -- sometimes Ubuntu and Mac OS X performed the same while in other cases Mac OS X 10.7.2 was faster with both GCC and LLVM/Clang.
Lastly, with the TSCP automated chess benchmark, the Clang-built version on Mac OS X 10.7.2 came in just behind Ubuntu 11.10.
As you can see, the performance results between Mac OS X 10.7.2 and Ubuntu 11.10 are definitely mixed, at least when using the latest-generation Intel Sandy Bridge hardware. One trend though is that using LLVM/Clang 3.0 within Apple's Xcode4 package these days is a much better option than using the GCC 4.2.1 release they have shipped for a while. Depending upon the particular workload you're interested in, you can run the given tests relevant to you under both operating systems using the Phoronix Test Suite with OpenBenchmarking.org to determine what platform is able to meet your performance needs, aside from any other software platform features to consider.