I bicycle commute in Dallas, and the DART rules are exactly the same as BART. I purchased a Dahon Matrix 2004 model. I have no complaints about the bike, I have owned it for 5 months and ride it every day. When folded it can easily be rolled. It is not the smallest package folded-- it can be made smaller by loosening the handlerbar bolts and swinging the bars 90 degrees--this won't save much room and is in my opinion more trouble than it is worth.
After much comparison, I finally went with a 26 because I felt more confident over less than perfect roads, and in general felt more comfortable on a full sized bike. If you are planning to only use the bike for short commuting, I would lean towards a 20 inch folder because of the easy portability. The Matrix is easy to roll around folded, but it is pretty heavy and cumbersome to lift. I can usually throw it in a friends' trunk if the situation arises, but it is never an easy job. I can't imagine being comfortable on the Dallas roads (not always the best quality and full of very aggressive drivers) without a full-size bike, but these worries may not be a consideration for you. Others may have more to add, but the main drawbacks I find to the 20 inch wheels are the decresed effeciency over long distance (though the smaller wheel will acclerate more quickly) and the diminished handling over less than smooth terrain.