Advice on using ASP.net WebForms or MVC

I have a public facing hobby site that gets about 3000 unique visitors a day, written in classic ASP that is in bad need of a revamp and redesign. I’ve faced the realization that an upgrade to ASP.net is the best way to go to implement features that are just too hard in ASP for the hobbyist (consuming RSS feeds, authentication and user profiles) but which I’m keen to get stuck into once I get past the redesign/upgrade.

Cannot pass complex object to another Action method

I’m trying to pass an object using TempData to another action method. Instead of redirecting to the action method, the controller gives me a white screen with the current action method in the URL. If I comment out the line where I pass the object to TempData, it redirects correctly. Is my object too complex to pass? Is there an alternative way of passing a complex object to another action methods?