I'll compare the products I've tried. M21 vs. DG105. M21 is much better on looks and I've gotten durability around 3 months or so (more on this in a second). M21 topped with M26 is a hard combo to beat on black for looks and durability about the same at 3 months (for me).
DG105 is a little dull in the looks department, but durability is insane. I've gotten about 6 months of durability out of a single layer of DG105. The 6 months is winter months parked outside everynight in the snow/ice/road grime. Compare that to the 3 months on M21 where the car was daily driven, but garaged every night.
I'm told that Aquawax on top of DG105 is basically a night and day difference in the looks department. I haven't tried it yet (but have some now), but DG105 + AW might rival M21+M26 regarding looks.