One thing I like about the BD is that the rear is built to the xtracycle dimensions, if I'm not mistaken. That means all the xtracycle accessories should fit. There is not one dominant standard in the longtail cargo bike world, but the closest thing to one is xtracycle. So a bike with those dimensions out back would be a plus to me.
It's super easy to remove/replace the wideloaders, etc, on an xtracycle. Takes seconds.
I agree with what a poster said above. The BD is probably a better bike more of the time. If you are hauling ultra heavy loads the majority of the time, I give the Mundo the nod.
Since you're building either bike with equivalent components, the end result price will only basically be the difference in frame cost up front. So is a complete BD worth $400 more than a complete Mundo? Yes, I'd say so. You're basically paying half the difference in what the bikes cost new, complete. A bargain if you build yourself and only have a $400 difference.
The Edgerunners look appealing to me. I don't know what a frame costs, assuming you can buy just a frame.
I don't necessarily buy bikes for resale value, but it is one small factor to think about. Selling a BD later would probably be a piece of cake, and you'd likely get good coin for it. I think you'd have a more limited audience for a Mundo and hence get less out of it later and have less people responding to your CL ad. But just my off the cuff opinion there, have not done the research to be sure. I think a BD build would be more desirable in my neck of the woods, anyway.