Personally I'd have to pick the harlequin...not only is it a much more colorful choice (in my opinion) but it is also a much more logical selection due to system requirements. The garabaldi requires a much cooler system than most hobbyists maintain...they will be extremely territorial and aggressive and may pose filtration problems. below is a quote from a article that focuses on the garabaldi and velvet damsels from Bob Fenner...
"the Giant (Mexican) Damsel (Microspathodon dorsalis, formerly Damalichthys vacca) and the equally gargantuan Garibaldi (Hypsypops rubicunda).
As juveniles these extra large damsels are dazzling (Volume 1 of T.F.H.'s Marine Atlas p. 397 shows the young and adult garibaldi, p. 408 the giant damsel). While you're there please note the other pictured "jeweled damsels pictured; the frequently offered Caribbean Microspathodon chrysurus, the semi-official 'true' jewel or neon-velvet damsel, Neoglyphidodon oxyodon, and the much less seen Plectroglyphidodon lacrymatus, both from the west Pacific. The Cortez jewel-markings as a youngster are easy to distinguish from the other purple-blue dotted species. Unlike the Caribbean member in the same genus, it lacks a yellow tail.
The number one problem with these two "big boys" is just that; they get too darn big. Oh sure, they're cute and oh-so active when tiny young. Ask someone who has been terrorized (yes, that's the right word) by a couple of inch domino while working in a marine tank. I've seen damsels draw blood. Gram for gram (ounce per ounce) they are amongst the toughest meanest turf fighters among reef animals. If they can intimidate an aquarist, imagine how their tankmates feel. Now just think how it would be if that three spot domino got to be over a foot long and two pounds in weight. Our two jumbo damsels do.
Secondly, the issue of temperature. The Garibaldi is a Baja and U.S. California local. It lives in water that spans the fifties and sixties of degrees Fahrenheit. The Giant Damsel hails from Mexico's Sea of Cortez on down to the middle of Central America on the East Pacific coast in water to the mid-seventies. Both these fish stress out at higher temperatures."
