REGULATORS ARE REGULATORS ARE REGULATORS.
every one that is sold for use for SCUBA DIVING has to pass certain regulations on their ability to provide air in an AVERAGE state.
The differences are as follows
1: construction material. Plastic, Polcarb, Carbon Fibre, Stainless, titanium. the list goes on.
2: size of the second stage; flight type for lightness and travel, large for gloved hands on the purge
3: cost of the internal parts. some are made cheaper with less positive wearing parts. (service costs are higher!)
4: First stage set up. Is it Sealed? DIN OR YOKE mounted?
So basically, you need to look firstly at the money you can afford.
then what kind of diving will you do?
if its in cold water as well as warm, then look for a set up that has an Environmentally sealed FIRST STAGE. this helps with freezing.
you then have PISTON over DIAPHRAGM first stage.
after that, whats the inter-stage pressures? ask the Shop about Flow and delivery of each you look at.
Make sure you match the first stage with the second stages. Dont mix them as Interstage pressures are important.
Basically then try them. If they feel comfortable, and deliver air nicely to you then youre halfway there!
After that, look at where to get it serviced. and what cost. some cheaper models will end up costing you the same if not more after 4 years of servicing, than the more expensive model!
In the end, buy the BEST you can afford, The most comfortable and the ones that you are happy with.