Depends what you want it to do. I wanted mine to eat algae. I've had both the tuxedo (Mespilia globulus) and a "colored" urchin (Tripneustes sp). The tuxedo cruises around the tank, eats algae here and there. The colored urchin ate coralline and made a mess (plus he grew huge). Eventually I took him into a LFS for credit as he was getting too big and knocking over corals.
The ultimate urchin (of those that are commonly available) is the longspined urchin (Echinothrix diadema/Diadema setosum). While the spines are sharp and somewhat venomous (to some, not so much to others). They certainly are gorgeous.
Also it would be dependent on your tank size as the diadema urchins get kind of big.
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