Consider the following:

• I need to fill Rebreather cylinders 1,2,3 and 4 litre & occasionally stage cylinders 6, 7, 10 and 12 litre.
• I want to use it when I am away from home at dive locations.
• I can take time to fill my cylinders between dives.
• I want to fly / travel with it.
• I don't want to spend too much money on buying the booster.
• I want low cost servicing and spares.
• I want reliability.

Then you need a ROB or an Expedition Booster.

Alternatively you may require the following.

• I need to fill 12, 15 and 18 litre twinsets with mixed gases within minutes, while the diver waits.
• I want a booster that can run 24/7 if required.
• I want a safe and reliable system
• I want to set the cycle speed and have minimum intervention.
• I want low service and spares costs.
• I want to rent less industrial gas cylinders and return them nearly empty.

Then you need a MIDI Booster.

Finally, if you have this requirement;

• I want to drain the supply gas cylinder to near empty and boost the target cylinder 300 bar in one go.
• I require a portable booster pump system to travel with.
• I can take a little time to boost cylinders between dives.
• I want low cost servicing and spares.
• I want reliability.
• I can afford to pay a little more for highly engineered specialist kit

Then you need a Two Stage ROB.

Finally, other boosters are available but consider

• The "original" gas booster with a 6" air drive are expensive to buy, with costly spares and service, and need a huge 3 phase LP air compressor to drive it.
• Some boosters have inherent design flaws, unreliable flap check valves.

So choose carefully.