They both had fresh batteries, and they are almost the same size with the Mars 3.0 being slightly bigger and heavier but not by much.
As for the batteries in the Mars 3.0 that is a problem. It has a bone headed design which indeed uses a tiny screw to fasten the battery compartment. It's an absolutely horrible design.
The mount seems quite good and is similar to the SuperFlash as a tab slides into the bracket.