Question: What is a good datatype to keep in session for a large collection of keys and values to frequently reference and update?

Application: Updating an old .NET web app with a million pages and grids to have all the grids maintain their sort. They currently access helper code to format themselves graphically on load and on sort. I figured I could add to that code to check for a key based on the page and grid id in a collection in session to see if it has a previous expression on load. and the on sort update/add its appropriate item in the collection.

Thoughts?

Dictionary vs NameValueCollection