If your interests are focused on doing more client-side development, I would go with C#.  If your interests are more focused on doing more back-end server development, I would go with Java.  If you go the Java route and really want to learn back-end development, then I suggest also getting dirty with Glassfish or JBoss and start absorbing Enterprise Java concepts.

If your interests are focused on doing more client-side development, I would go with C#.  If your interests are more focused on doing more back-end server development, I would go with Java.