All my comments should be taken with a grain of salt. I've been a Spring user since version 1.0; I've never used any version of EJB in production - from 1.0 to the present.
Spring has served me well: any technique that I would want to apply from Java EE is available to me in Spring. I've used the web services, LDAP, messaging, and portlet modules along with the usual suspects from persistence, remoting, and web MVC. The code is excellent. I find that emphasis on layering, coding to interfaces, dependency injection and aspect-oriented programming benefits my designs.
The truth is that you can choose either and make it work. There's little advantage or disadvantage at this point, because EJB 3 took so much from Spring that they're supposed to be similar. I can't say that from personal experience, because I haven't used EJB 3.
None of my employers or clients are clamoring for EJB 3. People in my market are using Spring or .NET to talk to web services over HTTP.
EJBs require that you use RMI or CORBA as your network protocol. I believe that HTTP based protocols - be it RPC-XML, SOAP, or REST - are winning, because simple and open usually does.
No one can foretell the future, so job prospect advice is worthless. Your local market conditions might not be the same as mine. You can hedge your bets and learn both. Whichever you choose, it's almost certain that you'll have to learn something new in the future that won't be either EJB or Spring. So pick one and be ready to change.
