As an aside, what units would you suggest people look at?
What
@rjack321 said basically.
The list is whittled down rapidly for most divers.
Question 1-do you need it to be sidemount? Do you need it to easily travel? Do you have any weird special considerations?
Example, for sidemount it obviously rules out everything except Choptima, SF2, Sidewinder, Sidekick, and Liberty. Nothing else out commercially on the market *yes I know that the Flex and T-Reb exist, no I do not consider them viable* Which one of those will depending on the rest of the questions. Weird considerations would be you're a GUE diver interested in projects, that means JJ or JJ, or you are involved in scientific diving which usually means APD, etc.
Question 2-are you currently or intend to be associated with a dive shop? If you answer "yes my shop is an SSI shop and a Mares dealer", then you should probably seriously look at a Revo or whatever applicable unit for the brands they currently sell. If you intend to teach on the unit, what unit will let you sell courses and units, i.e. if there is already a Meg dealer in close proximity then you probably shouldn't choose the Meg.
Question 3-are you bound to a particular instructor? That obviously limits the units to what they teach which is what
@Marie13 is restricted with at the moment.
Question 4-are your teammates heavily invested in a particular unit? If they are, then you should probably go with that one. The reality is basically all of the backmount units are perfectly good and most of them are functionally identical so if you really want a Meg but all of your buddies are on an X-CCR, then get the X-CCR because it means all of your spare parts are compatible and the units are all but indistinguishable when you're diving them.
Note the several times I alluded to all of the units being basically the same which means that there is absolutely nothing wrong with any of those restrictions trimming you down to a specific unit.