OK, here's one try. I am not the reg geek, but I can try to give you a non-comprehensive list of some things that differentiate an expensive reg from a less expensive reg. (1) High performance. This is not just some marketing lingo. "High performance" regs will deliver air more consistently at depth, particularly when cylinder pressure is low. (2) Breathing effort. Really good regs have a very low breathing effort. This is measurable. Scuba Diving Magazine's ScubaLab used to run tests. I stopped reading the mag after Rodale's sold it, so I don't know what they've done lately. I know that my Scuba Pro MK25/S600 breathes a lot easier than my MK20/D400, which is also an excellent hig-performance reg. (3) Adjustments. Some regs let the diver adjust breathing effort or cracking pressure. Adjusting for minimum breathing effort can make the reg more likely to free flow, so it might be adjusted in some situations. I have to confess, even though my reg is adjustable, I don't use the adjustments that much and don't fully understand the adjustments. I got it for size, light weight, and low breathing effort and the adjustments came along with it. (4) Size and weight. Some are bigger, some are smaller. Some are lighter, some are heavier. A lighter second stage can reduce jaw fatigue. I happen to like a more compact second stage, but it is not critical. (5) Resistance to Cold. If you dive cold water (like we do in the Great Lakes), you want to make sure your reg will resist freezing up in cold conditions. Some aren't designed for it at all, some are good in all but the coldest water, and some are designed to work for ice diving. (6) Quality and service. Some regs and manufacturers are very well-known for high quality, good warranties, and good customer service. Scuba Pro and Aqualung have been around forever and are always considered top quality. They're usually more expensive too. I hear lots of good things about Atomic regs (company run by former Scuba Pro engineers), and Zeagle. Lots of people like Mares, but I am not sure they have quite the same reputation for regs (BIG DISCLAIMER - I could be wrong on the Mares rep, this is mostly lack of familiarity on my part). (7) Availability of Parts and Service. One of the good things about Scuba Pro and Aqualung is you can usually get parts and service even if it breaks while you are on vacation. They are everywhere. You also want to get a brand that you can easily get serviced for the regular maintenance. Personally, I think this means buying it at a local dive shop you trust that has a good technician. Others may disagree, which is, of course, their right. (8) Retailer. If you have a good relationship with a local retailer, you may want to buy what they sell. They won't sell everything, but you should have some choice. There is enough good stuff out there that you should be able to buy something good from your local dive shop if you are inclined to buy there. (9) Special-purpose stuff. Some regs are particularly good for particular uses. For example, some first stages are environmentally sealed. This means no water enters the first stage, which makes them good for diving in contaminated or very cold water. Contaminated water shouldn't be an issue unless you are diving for golf balls, a commercial diver of some sort, or a public safety diver. For the rest of us, contaminated water usually means peeing in your wetsuit.
Well, that turned out to be more than I thought, but it is not the whole story by a long shot. I expect somebody else will come along to correct any mistakes and plug any holes. Hope this is helpful.
