How to choose the right regulator?
What to consider with first stages
First of all we have to distinguish between unbalanced and balanced first stages. Unbalanced first stages decrease the intermediate pressure (that affects the performance) while lowering the pressure in your cylinder during the dive. Balanced first stages are not affected of the pressure in your cylinder, the intermediate pressure stays the same and so does the performance throughout the dive.
However, there are reasons to choose an unbalanced first stage such as only using it for shallow dives and being in very remote areas. The second reason comes to servicing the stage, since the construction is much simpler it is also a lot easier to maintain it. One other reason could be the lower cost, although buying a regulator is a long term investment and usually it is well worth it to invest a bit more.
All the other reasons will speak for a balanced first stage. Looking at those, there are again two types we have to differentiate: piston or membrane controlled. Piston controlled first stages achieve the highest air delivery rates. However, since the air delivery rates of membrane first stages are by far high enough (with all the security margins needed) they tend to be the more preferred option. Not only because they are less complicated in construction, also because they don’t have any moving components having direct water contact.
What to look at with second stages
With second stages there are more facts which might affect the individual preference of a certain type. Diving in areas from the shore with lots of surge, technical diving with attaching the regulator around your neck could be reasons, why you want to choose an adjustable second stage. (where you can decrease the air delivery)
Diving in very cold water will ask for a high performance regulator (balanced first stage) with very good cold water characteristics. Such as choosing a material in the second stage which is a good heat conductor (the most sensitive part of the regulator for freezing is probably the piston of the second stage). Having a good heat conductor such as metal will help to prevent from freezing
Also a second stage (on the picture above) made of plastic can have a great performance in cold water. As you can see on the picture the connection between hose and second stage is made from metal and powered by cooling ribs to increase the temperature exchange. (Cold water is still warm compared to the freezing cold temperatures which emerge around the piston in the second stage. On that same second stage you also see the adjusting screw.
Which brand should I choose
This is a very personal choice and should be affected by the supply of your local market. Don’t buy a brand which no dealer in your area will carry. The best option is to buy it with your local dive shop, they will know what is best for the certain region or and the area you want to travel to. You will require service and maintenance, so having a local dealer you can go back to will help you in the future.