not sure how either one are with brown algae but if you have plants, I'd choose the otos since the mystery snail has a 50/50 chance of eating every last plant you have. KYle
I think I remember hearing that ottos are the whiz kids of the brown algae world. I can't remember for sure though.
Another option would be amano shrimp. They eat a lot of algae varieties and contribute very little to the bioload. Its been my experience that snails are not the world's greatest algae controllers. Especially just one.
Otos are excellent at controlling brown algae. I have 1 otocinclus affinis (at least I believe so) and 4 zebra otos in my 29 gal. They have eaten most of the brown algae already, so I will have to start supplementing their diets. The zebras are pretty large as otos go, and they sure are active algae eaters!
If you buy them at an lfs, be sure to pick out healthy specimens. They should be fairly active, with full rounded bellies.
Rather than getting a fish to control the brown algae, have you tried to eliminate the majority of it? Brown algae is commonly found in abundance with newer tanks or tanks with too much light, or when fish are regularly fed too much food.
I guess what I'm saying is, in short... brown algae can be controlled easier than other kinds just because of the fact that brown algae is usually caused by a side-problem.