No idea about Paradise Down but....As a newbie myself, I have to say Belize Diving Services. The dive shop is a bit off the beaten path (ie: less tourists), the owners are right there on site and are *very* concerned that you have a good dive experience.
As well, the other dive shops take you out ont he local reef for your dives whereas BDS will take you wherever they happen to be going so you may end up doing you Open Water dives at Turneffe, Long Caye, Hol Chan, etc etc.....
Lastly, I was a very nervious newbie and the people at BDS went out of their way to make sure I was comfortable, well taken care of, and OK with what they were asking me to do.
They are worth a visit and in my opinion, worth at least a visit.