Kept thinking I should grab some pics of my urchins... But lights just went out - maybe tomorow.
Depends what you're looking for - if you want a true algae eating urchin, definitely look at the diadema's. They can be found VERY small at the stores on occasion (under 2" with spines). Mines doing an amazing job in a badly overgrown 3g, he's roughly 1", all the rock was covered in some kinda nasty hair algae - about a month later, it's still roughly 1/3 covered - but definitely getting much better. He's actually quite graceful moving around the tank, ****es off the zoos trampling them - but no permanent damage.
Personaly I'm not fond of the short spined urchins (tuxedo and that other one), more their habit of decorating themselves with corals
I picked up what I think is a Echinometra species (might be E. la****er - rock borer urchin) from howdy last trip to edmonton. Not real usefull for algae control (except coraline) pretty much the typical 'bull in a china shop' type urchin. But he's sure an interesting character in the tank, always fun to see where he is on any particular day, and what he's getting into.