I a have been working at my job for around a year doing database and development work and we currently use sql server 2005, which you think would make my decision easy to pursue a certification in sql server 2008. However, i have recently been looking on a few job posting websites and it seems like there is a bigger market for oracle dba's.
My anecdotal observations are that Oracle dba's make more because there are fewer good ones. I believe this is because Oracle costs so much that larger organizations use it a lot more than small companies, in general. Those large organizations tend to want more experience, and pay more for it.
SQL server, on the other hand, is more readily available, even in very small organizations. Microsoft is EVERYWHERE. So even newbies have a much greater chance to be exposed to SQL Server. Newbies get paid less, so they bring the average down. Don't get me wrong, you can still get to the point where you will make a lot doing SQL Server, but I think it is harder to find the ultra-high-paying jobs since the market has so many more Microsoft experts.
Long term, for the money, go for the Oracle certification, but combine that with some real knowledge...don't be a paper cert. Pay for it yourself, or self-study if you have to...it is worth it.
Many employers will pay for staff to train and certify in relevant products. Your current employer might be reluctant to pay for certification in a product that you don't use!
To put this into context, to complete the Oracle 11g DBA certification path to OCM level you need to participate in three instructor-led training courses, each one costing up to $5000 dollars and five days of your time, plus the cost (and time) of taking the exams.