Today I made an even trade of my Aqualung LX for a Mares Abysis - my LDS was great; I'll give them a plug; Austins Dive Center in Miami -
They have been in business since the '50's, have very good prices and I have done business with them for over 20 years.
Comment from the manager was "we just want to see you happy...." - the LX was 2 months old and used for over 15 dives - and they swapped it with no hassle, restocking fee, attitude, etc.
I don't know any mail order service that would let you do that - even Divers Direct, a great outfit I have recently purchased from, has a 30 day return policy and the return is subject to restocking fee.
If you are new to diving, or wanting to purchase new equipment, and are unsure about its sutability, my vote is for the local shop - if they offer a return policy.
I'm diving tomorrow in Jupiter, so hopefully the reg does better and cottonmouth is a bad memory!
Thanks again to all who replied.