Each shadow handgun type, and by extension most handguns, have different purposes they're tailored for and ultimately perform extremely well in. The primary exception is Valiance, which can be quite literally used for anything.

As a general breakdown:

Antigua line (AP/Sentenza) handguns are best suited for making quick shots at things, such as breathing zombies or retreating Thwackers, or sniping enemies who abuse distance, like Kats and Gremlin Demos. Antiguas also excel in mobile, consistent single-target DPS without any fancy gun switching, just mash attack until something dies.

The main attraction of an Alchemer is the charge, which is used primarily to eliminate or heavily damage a crowd of enemies, or deal a large amount of damage to a single target. An Alchemer charge will destroy a crowd of zombies, gremlins, slimes, depending on what you're shooting at. The bullets move at medium-fast speed, which will induce Gremlins to dodge, but also split. With a technique known as gun-swapping, you can actually achiever higher mobile DPS than an Antigua line, but you'll need at least 2 Alchemers to gun-swap effectively. Prismatech becomes absurdly powerful at higher levels, a charge attack will more or less annihilate anything vulnerable to Elemental damage.

Pulsar series are best used as utility weapons, to give you and your party breathing space in the event of a sudden incursion of enemies in your face. Polaris is better at shutting down turrets, due to the elemental damage and shock status, while Supernova is better to keep enemies in general away from you. Both can and will piss off swordsmen in the party, so if you get a Pulsar, use it sparingly.

There's more crap that I don't feel like writing unless you really ask me. I'm no professional gunner, so you might want to consult another person for a better opinion.

swaps, double swaps
http://www.youtube.com/watch?v=o4F0TNTbJvY

double swap cancels only require 1 attack weapon, so you only need 1 alchemer.