...and I'm sure someone could prove statistically that power mods work out better in the long run ... *shrug*.
For the sake of simplicity, I'm going to run this on the assumption of using only one gun. Once we deal with DG and mods, things get more complicated due to the fact that mods only apply to one gun. I can do a complete analysis if you want, but this post will stick to a quick rundown so you get the idea of things.
Abbreviations:
DMG - damage (hereafter always considered by the average)
FT - firing time
RT - reload time
CS - clip size
DPS - damage per second
Pistols have the following stats: 5 DMG, 1.0s FT, 1.2s RT 6 CS. For a single pistol's DPS over a long period of time, we do the following calculation:
DMG / (FT + (RT / CS))
Plugging in our stats, we get 5 / (1.0 + (1.2 / 6)) = 4.167 DPS.
A single power mod changes the pistol's stats to: 6 DMG, 1.0s FT, 1.2s RT, 6 CS. Using the same calculation, we get 6 / (1.0 + (1.2 / 6)) = 5 DPS.
A single bulk mod changes the pistol's stats to: 5 DMG, 1.0s FT, 1.2s RT, 8 CS. Using the same calculation, we get 5 / (1.0 + (1.2 / 8)) = 4.348 DPS.
Since they are additive, additional power mods will add 5/6 or ~0.833 DPS (thus P2 gives 5.833 DPS and P3 gives 6.667 DPS).
Additional bulk mods will add less DPS after each mod, with an asymptotic maximum of 5 DPS (though practically, B2 gives 4.464 DPS and B3 gives 4.545 DPS).
So yeah, for sheer damage, power is the way to go. This sort of analysis doesn't take into account player tactics, however. Sometimes it's better to have a bigger clip, because you may have a way of safe or instant (read: MGK) reloading.