I say the mech test its more for completests ..up against the dx joker, joker wins as he is a ..how to put this? ..a finished character? ..mech test is based on a scene from the first ironman movie where as joker is ..well the joker as he appeared for most of the dark knight.
I can see that, I also think that as a collector I try to go for more Iconic pieces, currently I am heavy into the GI Joe line because it is a fond memory from my childhood where as Mech test is just a cool guy from a movie and moreso the DX Joker is a cool character from an OK movie.