first, struts is not compatible with portlet unless you use the struts bridge with allow struts app to run within a portlet.
this can be usefull if you want to reuse an application as it is. however you will not leverage portlet (like user preferences)
if you start a new development, you really should not use struts as it is not natively compatible. for the MVC paradigm, my answer is that portlet provide already a implicit paradigm.
There is no equivalent to struts so far, the only framework existing working in the portlet env is JSF.
My advice is, start do to your application using just portlets and learn the API, then if you want learn JSF or come up with your own MVC framework leveraging portlet instead.