Everytime I have had my as blennies or know people that have, they seem to become more reclusive overtime. To begin with they are always out swimming but once settled mine find a nice hole and spend most of the time there. The one I have now, I hardly ever see as it has a hole at the back of the tank and the food goes right past it when I feed.
my experience of fang blennies only relates to the striped but it was always out centre stage and slowly moving about. So whilst I don't think with either choice you will lose out (midas slightly more comical but fang more out there) I'd probably go for the fang.