The 9mm is the pistol Deacon starts with, and it is the weakest sidearm in the game. It only ever comes in Junk condition. Swap it for the SAP9 at Hot Springs as soon as you can, since that costs just 500 credits.
It works for early stealth headshots on Swarmers and lone humans, especially with an oil filter suppressor. Body shots take several rounds per kill, so it burns ammo fast in a real fight.
The SAP9 is the first upgrade and sells at Hot Springs with no Trust needed. It holds 12 rounds and aims better. Later the PDW and Eliminator leave both far behind.
Eliminator
The Eliminator is a Military Spec heavy pistol that can drop a Freaker in one shot. Jacob D'Angelo sells it at Wizard Island from Trust level 2. Expect heavy recoil and 9 rounds a magazine.
PDW
The PDW is a Special Forces sidearm with high accuracy, low recoil and a 20-round magazine. It costs 1,250 credits. It is one of the best pistols in the game, rivaled only by the Eliminator.
SAP9
The SAP9 is a Poor-condition pistol sold at Hot Springs for 500 credits with no Trust needed. It is a clear upgrade over the 9mm, with a 12-round magazine and better accuracy. Buy it on your first visit.
SMP9
The SMP9 is an automatic sidearm with a 30-round magazine. You can buy it at Hot Springs or Lost Lake, or get a better Mil-Spec copy free from Horde Killer. Full-auto in the sidearm slot is great on the bike.