The game is biased to prefer fast guns:
The damage/shot + shots/second of guns are “balanced” (details) to be around 1.6x your loadout attack (some a bit less, some a bit more). However,
- Gear damage adds on per hit, which is more helpful for fast guns than slow (especially as your Gear Mastery permanent upgrade levels up).
- Likewise the fixed bonuses (+100 damage against Ground enemies, etc) also add on per hit and are more helpful for fast guns
- Ditto the attack boost you get from Mystery crates, especially in endless / endless boss / Dark Castle where you rack up many of them
There are several perks that work better with fast guns
- Focus Shots, because the damage increment is per hit, and it scales up faster for a fast gun
- Ice Shots and Improved Ice Shots, because the damage is per hit
- Headshot, because the time saved when it procs is greater with a fast gun than with a slow one (which might be one-shotting mobs anyway)
- Zap shots and Improved Zap shots, because the Zap damage is per hit and is a percentage of your nominal damage per shot, and the additional benefit of gear damage (and attack bonuses from crates) for fast guns carries forward to zap shots as well
Several heroes have skills that proc based on hits, which means a fast and weak gun that needs more shots to kill an enemy will proc the skill more often than a slow powerful gun:
JackBot’s Jackpot, Mortis’ Plague Cloud, Fursus’ Iceplosion, Prospero’s Magic Bullet and (I think) Ignus’ Flame Shield will all perform best with the looper (because of all the indirect hits from piercing and boomerang), and 2nd best with the Smart Minigun.
In addition, Arctor’s Freeze generally takes 3 hits to freeze, so will freeze faster with a fast gun. Blazia’s Wrath will also work a little better with a fast gun
On the other hand, the only situation where the game benefits slow guns is the Revenge perk, but this perk has limited utility because in tough runs you can’t afford to get hit multiple times