The AT20 second stage will most likely come with a US4 or a DS4 first stage. The US4 is identical to the DS4 except that the DS4 is environmentally sealed and rated fro cold water < 10 degrees Celsius.
Both US4 and DS4 first stages has 4 LP ports and 1 HP port. These first stages have a good reputation amongst Tec divers. They're easy to service. They're lighter than most first stage variants and are great for doubles. You can also use them for deco bottles (lots of divers do so) but note that they're non swivel. This means that your 40' deco bottle hose will a bit tight when you use them over your neck during a gas switch. A 42' hose will do a nicer job.
The second stage AT20 has a venturi adjustment lever. They're easy to service and unlike higher end Apeks second stages, you can easily open the front plate with your hands to dislodge obstructions in the diaphragm.
Both US4 and DS4 first stages has 4 LP ports and 1 HP port. These first stages have a good reputation amongst Tec divers. They're easy to service. They're lighter than most first stage variants and are great for doubles. You can also use them for deco bottles (lots of divers do so) but note that they're non swivel. This means that your 40' deco bottle hose will a bit tight when you use them over your neck during a gas switch. A 42' hose will do a nicer job.
The second stage AT20 has a venturi adjustment lever. They're easy to service and unlike higher end Apeks second stages, you can easily open the front plate with your hands to dislodge obstructions in the diaphragm.