Here you can find white paper from Seal Rock Research which "provides key facts to help customers choose between Linux and Solaris for x86/Opteron servers". While it doesn't go into very details it's interesting to read.
Below are conclusions and recommendations which are Copy&Paste section from this document although I suggest to read whole white paper.
Conclusions and recommendations
Either Solaris or Linux can fill almost any enterprise role.
Linux is and will continue to be an excellent choice in many server roles. Its steadily improving performance and capabilities mean that Linux will maintain its momentum among ISVs and customers.
Solaris is superior to Linux in some important areas. Decision makers who are not constrained by application availability or hardware support should seriously consider Solaris for deployment on x86/Opteron servers.
Organizations contemplating a switch from Solaris on SPARC to Linux can maximize the value of their existing IT skills and infrastructure, while minimizing the risks and costs of moving to a different platform, by adopting Solaris on Opteron or Xeon servers.
Organizations that have found Linux to be more expensive or less satisfactory than expected may find that Solaris offers an effective alternative.
Organizations without a significant investment in either operating system can compare the benefits and risks of these excellent alternatives.
For custom applications, choose Solaris. DTrace, the Solaris Application Guarantee, and Solaris' unparalleled stability will protect your investment in development and make it the better choice.
If you want to run commercial applications, you may have no choice. Linux leads in ISV support on x86 platforms. But check with vendors before buying they may have a Solaris version in the works.
For open-source applications, the choice is more difficult. Solaris offers Linux binary compatibility, and most common open-source applications are readily available as native Solaris binaries. But many open-source projects are based in Linux. This means that installation and configuration may be easier, and community support more readily available. This may trump Solaris' superior security facilities and overall lower cost.