It's a subtle thing. The price point is different because the complexity of the valve is less with an unbalanced second stage. An unbalanced second stage is more susceptible to differences in tuning . That is, the valve opens easier when the pressure coming from the first stage is higher, and becomes more difficult to open as the assistance from the first stage gets lower. A balanced second stage tends to respond to changing intermediate pressure (the pressure from the first age), and will be more stable.
This plays out in several different ways. If you don't tune your regs regularly, an unbalanced second stage may either become stiffer or may start to freeflow as the rubber seat ages and takes a groove.
Depending upon the composition of the seat, a balanced second stage may remain stable longer. This may be significant for those "overbalanced" first stages like an Aqualung, that see an increase in intermediate pressure as the tank empties. An out of tune unbalanced second stage may begin to freeflow at depth with a higher IP.
On the good side regarding unbalanced seconds is their simplicity. Some technical divers will use an unbalanced second stage, and just keep them in tune regularly. If they freeflow from a grain of sand caught in the valve, a simple bang on the thigh may fix things.
As for making a choice, my recommendation would be to choose balanced firsts and seconds , even though there is some redundancy there. The reason is that they often are more stable in their breathing characteristics for a longer period of time. If you have a specific reason to choose an unbalanced second, or if price point is a significant concern, they remain excellent regulators. One thing I would not do however, is use an unbalanced first stage, with an unbalanced second. This is just too old-school, and will result in a noticeable increase in breathing effort as the tank drains. This was thought to be a good warning in the old days, but that was back before SPG's. Nowadays, if you are low on air, you don't want to have to be breathing harder for it.
Unlike balanced versus unbalanced second stages, balanced versus unbalanced firsts are very different. An unbalanced first stage is less expensive because of the technology, but also because of the performance. You may not be able to notice it on the average recreational dive, but the capabilities of an unbalanced first stage drop as tank pressures get lower and the intermediate pressure falls. Additionally , the inherent design is not capable of providing the high flows that top-of-the-line balanced first stages provide. Again, this is often only noticed on an ANSTI machine
The ANSTI Machine: Evaluating A Regulator's Breathing Characteristics , but it is still a real phenomenon that may have an impact at depth. Unless one is trying to save money by purchasing an unbalanced first for a pony tank or a deco tank, today's competitive prices tell me that I should go for performance. Virtually all of my first stages except for my pony bottle are balanced.
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