The mazzer mini has a great doser - sweeps clean and as long as youre careful with your "thwacking" (definitely not the technical term ...) its clean enough.
The $400 issue is a tougher question to answer. I think its easier to justify if youre more experienced with grinder adjustments and want to extract the last tenths out of an extraction - the Rocky with its steps can be a little easier to setup but of course doesnt allow the fineness of adjustment. The difference is probably more pronounced if youre a long black or espresso drinker and then even more so if you have an espresso machine that can get the best out of the grind (e61 group head, pre-infusion ...).
A taste test would be the only way to be sure whether its worth $400 to you.