A wise man once told me that the only general answer that suits all good questions, is the phrase "that depends." In the case of your good question, that depends on whether you want to be a Windows sysadmin or network administrator, or if you'd rather walk the Unix (or Solaris) side of the street. I can't really say one is better than the other, without understanding more about your career goals, experience, and interests.
In the absence of such information, let me point out that pursuing either certification will take time, cost money and require considerable work and effort. Since those facts are indisputable, choose the credential that most engages your interest and enthusiasm. Those ingredients will help you survive the certification experience and ensure a better education along the way.
If you just don't know which way to go, consider that there are four or five times as many companies or organizations that use Windows for networking than those that use Solaris (but consider also that four out of five of the biggest Web or Internet installations use Solaris for enterprise level services rather than Windows). Like I said, it depends on what you want to do, and who you want to work for...
This was first published in February 2002