It depends on the type of 'algae eater' - hopefully not a Chinese algae eater ! (too aggresive) - Plecs are good if you have the tank space ... but you need lots ! (although most fish shops will happily sell you small Plecs without asking about the size of tank) a 3 or 4 inch Plec in the fish shop can easily grow to a foot long over a few years. I would always go for Otocinclus (dwarf suckermouth) (Just my personal preference)
I have 3 Otocinclus in my 30 Gal tank and they do a great job of clearing away the algae.
They are especially good at cleaning algae off plants without causing any damage to the plants themselves, even the most delicate plants are left unharmed.
I find these fish better than Plecs, a fully grown Otto is only about 2.5inches and I have found that Plecs eat less algae as they get older/bigger, but the Otto's just keep on grazing on the green stuff (and the brown stuff).
Combine that with the fact that they dont produce much waste (unlike Plecs) and are quite active (when kept as 2+ ).
Just be sure to get good healthy ones from your supplier, they should have a 'pot belly' otherwise you could lose them soon after adding them to the tank.
Mr G