The Walther's fast reticule means you don't have to switch back and forth to your pistol, which makes life a lot easier. If you tend to fire through your clip very fast the Dragunov large 10 round magazine is an alternative choice. Because of their underpowered rounds and semi-auto fire you will only be using pistols on two occasions. The first is if your main weapon is a sniper rifle and you need to do some close quarters combat. The second is if your engaged with an enemy at short range and you shoot your main weapons magazine dry.
All pistols have incredibly fast reticules.
The differences between them are firepower and magazine size. Always carry an unsilenced version of the pistol you want to maximize handling and firepower. 9mm - Very weak, don't use this one against medium armor or higher. 15 round clip. USP - A in between caliber, slightly more powerful than the 9mm.