Upgrade pools
Each weapon has its own short list of stats, and every level-up rolls its upgrade from that list. A Flat entry adds a fixed amount, so Sword's Damage +2 is two more damage per hit. An Addition entry adds a percentage, like the Sword's Size +20%. Pools run from 2 stats to 6. The Revolver's six include Crit Chance, Crit Damage and Projectile Bounces, which gives it more ways to roll well than a four-stat weapon like the Sword.
Cooldown and base damage
Base damage only tells half the story. Sword hits for 11 every 0.75 seconds, Revolver for 5 every 1.4, and Flamewalker has no cooldown at all because it leaves a constant fire trail behind you. Weigh damage against cooldown when you pick.