It's all kind of dependent on what your potential employers are using at the moment, or are moving towards, isn't it?
According to the data I've been able to find (mostly gleaned from a couple of reports and press releases), it seems that WebLogic might well be the most prevalent, followed by WebSphere, so that might influence your choices. I'd also take a look on Monster and other job sites, and see what the employers are looking for to help guide your aim.
Theodore Jonathan Casser
SCJP/SCSNI/SCBCD/SCWCD/SCDJWS/SCMAD/SCEA/MCTS/MCPD... and so many more letters than you can shake a stick at!