Let me ask a few things, how often do you plan on diving annually? What kind of diving is expected? How many dive sites, stores and divers have you talked to about the specific item your looking at purchasing?
Choosing equipment is unique for each individual. The first question I normally tell my students that if they find themselves diving more than 10 times a year then go ahead and purchase your equipment. Not only is it yours and as such better cared for that some rental items you are safer as you become more comfortable in it, knowing where everything is at and now to use it. Important safety stuff here and a comfort level you may not have with different rental gear each dive.
Try before you buy! Most dive shops are going to let you do this and some will allow you to take some or all the rental price off if you decide to purchase.
Do Your Homework! You cannot research enough, talk to people, shops, people on the boards here, scubaboard and the like. While it may be ok to purchase some thing on line e.g. wet suits; others should be purchased from a shop that services them and have the technical staff to do so.
Lets say your reg starts free flowing a week before the dive trip to the Keys and there is a part needed. If you purchased it from the local dive shop chances are they can fix it for a valued customer. Cant do that with .com Who knows even if they cant fix it they may offer you one of theirs or stick on another second stage just to get you on your way to your trip
Additionally consider what your diving may be like in the future e.g. dont settle for an air computer, go nitrox; try the backplate and wing as it is more adjustable for a variety of dive types rather than a jacket BC.
Then take a while and choose wisely as we are all looking for the next customer and I like mine and want to keep them coming back.
God Bless and Dive safe!