Dual gauge single stage just has tank pressure and outlet pressure gauges and only reduces the pressure once.
Two stags as it sounds reduces pressure twice. This 2x step creates a more steady output pressure regardless of tank pressure.
The inter-stage pressure has no gauge but goes something like this:
900psi....200psi.....X psi (30 plus for atomizers, a few psi for reactors)
single stage (unless err "design modified") the outlet pressure increases as tank pressure decreases regardless of setting on the outlet side.
For CO2 this only impacts the end of the tank as tank pressure remains stable till all liquid CO2 is used up. Then tank pressure rapidly (for systems w/ higher output settings) drops.
Some think this could lead to an "end of tank dump" of CO2.
Personally I believe it can but isn't very common. Need to be running CO2 high enough to be border lethal and have a high flow rate so pressure increases a lot in a short period of time.
CO2 levels can then become lethal to non-plants.
Regardless, standard practice for single stage is change out the tank when pressure starts to drop since there is little CO2 left anyways.
Some think it's due to bad needle valves though.
2 single stages in series accomplishes the same thing as a 2 stage.
No opinion on your 2 choices though since many (inc. me) DIY 2 stage setups but costs can (and usually) do go over $100 ($200 is fairly easy) and of course used parts or new old stock and sweat equity involved.
Using commercial brand new parts one can come close to under $300 w/ some sweat equity (need to be a small plumber).
Obviously one needs to consider ones time as zero cost.
Atomizers are like high pressure air stones w/ tiny bubbles.
In-line or in tank.
Reactors just dribble CO2 into a mixing chamber to completely dissolve it so no bubbles.
There are other delivery systems but those 2 (3) are probably the most common.
Reactor is the best but involves more plumbing and parts usually. Those to are done DIY.
some sump/CO2 chatter.
Just found this..I know simplistic but it does explain it easily enough..