The politically correct answer is that both Windows and Solaris (given equivalent hardware) will perform equally as well. In my opinion as a TAC engineer, I prefer Solaris. I find it's much more compartmentalized, and easier to troubleshoot. Solaris server also don't require anti-virus software which generally means there is more memory and RAM available for LMS.
All that said, you need to pick a server platform that meets your company's needs and expertise. Don't go with Solaris if no one in your organization can manage it, optimize it, and keep it patched and secure. If your admins know Windows, and can effectively optimize, patch, and troubleshoot it, then you should go with Windows.
