Biz class on the train to BWI? Unless you're picking the Acela (where Biz = standard), it's hardly worth the upgrade for a 20-minute ride. You'll probably find it easiest to take the first available train anyway.

Actually, I get a kick out of the MARC runs where BWI is the first stop - how often are you on a commuter train that's moving along at a sustained 90 MPH+ clip?