There are two schools of thought.
1. The backup should be dirt simple as that makes it more reliable.
There is some theoretical merit to this but in my experience, if a second stage is properly maintained there is no difference in reliability between a higher end balanced and adjustable second stage like a G250HP or S600 and a lower end unbalanced and non adjustable second stage like an R390.
In addition, one of the most common problems enbcountered in the field is a slight freeflow caused by the LP seat in the second stage taking a set as it breaks itself in and/or wears a bit. With an adjustable reg, a half turn on the knob will resolve the problem, while a non adjustable reg requires an adjustment to the orifice, and unless you know how to do that yourself, it means an aborted dive and a trip to the LDS.
Furthermore since unbalanced regs like the R390 have to use a stronger spring in the second stage to overcome the downstream force of air from the first stage (which in turn continuously puts more pressure on the LP seat, causing more wear) you are more likely to encounter this problem with an unbalanced second stage. So on balance, I think the relaibility edge goes to more expensive balanced and adjustable regulators despite their slightly higher complexity and higher parts count.
2. The back up should be of the same quality as your primary.
This makes a great deal of sense as in most cases in an OOA situation, you will be breathing off your backup second stage and it obviously needs to perform at the same depths and under the same demanding conditions as your primary.
Mixing brands:
This does not present a problem functionally as nearly all currently produced regulators operate within the same Intermediate Pressure range of 120-145 psi. This means just about any second stage, regardless of brand, can be adjusted to operate with just about any first stage.
Where it can get interesting is getting them serviced if your LDS of choice is not a dealer for both the brands involved.
1. The backup should be dirt simple as that makes it more reliable.
There is some theoretical merit to this but in my experience, if a second stage is properly maintained there is no difference in reliability between a higher end balanced and adjustable second stage like a G250HP or S600 and a lower end unbalanced and non adjustable second stage like an R390.
In addition, one of the most common problems enbcountered in the field is a slight freeflow caused by the LP seat in the second stage taking a set as it breaks itself in and/or wears a bit. With an adjustable reg, a half turn on the knob will resolve the problem, while a non adjustable reg requires an adjustment to the orifice, and unless you know how to do that yourself, it means an aborted dive and a trip to the LDS.
Furthermore since unbalanced regs like the R390 have to use a stronger spring in the second stage to overcome the downstream force of air from the first stage (which in turn continuously puts more pressure on the LP seat, causing more wear) you are more likely to encounter this problem with an unbalanced second stage. So on balance, I think the relaibility edge goes to more expensive balanced and adjustable regulators despite their slightly higher complexity and higher parts count.
2. The back up should be of the same quality as your primary.
This makes a great deal of sense as in most cases in an OOA situation, you will be breathing off your backup second stage and it obviously needs to perform at the same depths and under the same demanding conditions as your primary.
Mixing brands:
This does not present a problem functionally as nearly all currently produced regulators operate within the same Intermediate Pressure range of 120-145 psi. This means just about any second stage, regardless of brand, can be adjusted to operate with just about any first stage.
Where it can get interesting is getting them serviced if your LDS of choice is not a dealer for both the brands involved.
