If you're headed to St. Louis, check out the Katy Trail --- I have heard of at least one cyclist who ventured that way as an alternate to the steep and rolling Ozarks in southern Missouri.
Depending on your definition of "Spring", the TransAm might be more comfortable (depending on your preferred temperatures). If mid-to-late May, either North or TransAm would probably be more comparable.
I'd be interested to hear your goals for westward from Colorado too --- if time was not an issue, you might follow the Trans Am/Northern Tier to the Pacific, then down the coast. However, a more direct route would either be the Western Express and/or Grand Canyon Connectors with Route 66 -- these routes might get hot by the time you arrive though!