get the largest one you can afford, even if it means putting some money aside for a few months.
sound reproduction wise, the larger the sub, the better it is for reproducing the whole freq. range.

a good sub is like a solid foundation under a house.
it lets you hear more of what the producer intended to convey.

See: http://www.blu-ray.com/community/gallery.php?member=Gelli
or: Axiom Gallery