You're comparing apples and oranges. Struts is for view and controller, not model processing. Especially when combined with Tiles, Struts will give you most of the portal-like functionality (rectangular placement of items with individual controllers behind them) out of the box.
However, that still doesn't address your model logic. That can be done with either EJB's or straight
Java beans (plain old java classes) or a combination of the two. Servlet's aren't appropriate here -- Servlets are also meant for controller logic, not model logic.
This is just standard MVC layering -- I'd strongly suggest you read a book that covers both Struts and Servlets so that you can better understand the differences between the two and the advantages/disadvantages of both.
Kyle