Depending on where you get the rock, mantis may or may not be a problem. I've had a few, but they've all been cooperative with their removal. But, rock really is better for the fish--provides cover and natural food sources.
I don't use a wet dry. I have 55 gallon sumps on my 120's, then lots of rock and sand, and as many cleaners as the tank will support--in the case of FO that's pretty limited, so a bit more manual cleaning is required. But, there's no mechanical filters on my setups to catch and trap solid wastes where they can decay and increase nitrates. I have the ability to add mechanical/chemical filtration if needed, but so far it's just been on stand by.