KUJayHawk
BSDI is a company based in rural PA outside of Bethlehem. The company has no website and the program is kind of a black box. It is not so much a software package as it is a service agreement for the guys at BSDI to run your model for you, send you the run data, finalize your model and give you advice on how to improve the design. You need to contact them directly and contract their services. They will collect information about your structure and develop the model for you and then send you interative results. You will optimize your section based on these results and send back for the subsequent iterations. The guys at BSDI work with the designer to ensure the kinks get worked out of your model and they can(and will) customize the program for you to fit the need of the project. It is a tru 3-d program, but you the Engineer do not have direct control over the model 100% of the time.
I cannot speak for MDX but DESCUS is more of a pure design program for steel plate and box girders. It can be customized to fit project needs if you contact the customer support people but only if you have your maintenance contract in place.
That being said, while BSDI is better than MDX or DESCUS as a method of developing a 3-d model of your bridge, it is not for your evryday structure and you are not in direct control of your model like you are with the ohter programs. The BSDI team is great though and it is worth giving them a call if you think you need their services.
Hope this helps.