I have worked in both Spring and Java EE 6. What I can say from my experience is that If you are going for the age old JSP or proprietary Flex then you are safe if you stay with Spring.
But if you are to move ahead with JSF then it's time to shift to Java EE 6. With Java EE 6 you are moving to Facelets and standardized script libraries and component libraries. No more script incompatibilities and component library matrices.
Regarding Spring MVC, it's good as long as your project doesn't grow too big. If it's a huge enterprise application stick on to Java EE 6. Because that's the only way you could maintain your own component libraries and resource bundles in an orderly manner.
