Well, being in the BEA camp, my first instinct is to just say BEA! But that's really not objective, is it?
I would say to get whichever your current employer is using. If you're unemployed or thinking about moving, then it's a toss-up. I say this because whichever one you study for will help you with the other. Both are equally as marketable, and both will give you an equal understanding of
JSP, EJB,
Servlets, Web Services, etc. - it's only the administration (and the container-specific deployment descriptors) which differ.
I was an IBM Notes/Domino developer before moving to
Java. Each product has its market base. You may also check out the demand for WebLogic vs. WebSphere developers at Monster.com.
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Joe McGuire
Sun Certified Java™ 2 Programmer, BEA WLS Certified Developer