To some extent, your question is like asking whether vanilla or chocolate ice cream is the best flavor. Some people like chocolate better, others vanilla, and that's really all there is to it. In terms of market share, Oracle rules at the enterprise level (but Microsoft has a bigger presence even there than many people realize), and Microsoft rules at the small- to medium-sized company level. Since jobs at the latter outnumber the former by at least 20 to 1, there may be more opportunities for SQL Server, but they won't pay as well as Oracle profesional jobs. But other factors come into play here, such as whether you're more comfortable working in a big corporate environment or whether small and medium sized companies are more to your liking.
My advice is to find some practiciing professionals on both sides of this street, and get them to tell you what their professional lives and experiences are like. Then you can go with the side that appeals to you most.
This was first published in November 2004