i've kept both fighting conchs and queen conchs. fighting conchs do stay smaller (4"-5"), but the queens do grow quite slowly. ours went from about an inch to 4.5" in just under 3 years (lost them in a tank crash). the fighting conchs can't climb and can only feed off of the substrate or as far as their proboscis can reach. the queens, however, CAN climb (don't ask me how they pull it off, but they do), and can be pretty darned clumsy, so if you have any loose frags or small rocks, they'll get bulldozed.
that being said, i have found both species to be great, and both will even eat red cyano, altho their preference is diatoms and film algae.
HTH