If it's up to you, you really should be looking at JSF, especially if you're using IBM tools. IBM has really let their whole Struts support slide, as that is no longer their direction. IBM is all about JSF now, and for that matter, JSF apps slide into a portal framework much better than Struts apps do.
Either way, using Hibernate or Spring within the portlet is completely fair game. Absolutely no problem using Hibernate with Portlets, or Spring for that matter. The only issue you might get is classloader issues, with potentially different versions of common jar files on the WebSphere classpath, but that can usually be rectified by configuring some of the PARENT_FIRST or PARENT_LAST classloader configurations.