Comparing a rifle with a cannon at rating 104:
- Rifle min damage is ~95% of cannon.
- Rifle max damage is ~75% of cannon.
- Rifle avg damage is ~75% of cannon.
Not knowing exactly how the game 'rolls' a value between the min and max range, I am just guessing that average damage is (min+max)/2.
I can't test cannon vs. rifle much further than this, as my troop is a Vanguard.
Higher rating weapons do cause the non-'weapon damage' abilities to do more damage, but does a higher weapon max damage affect your non-weapon damage abilities? If not, then it's really only 'weapon damage' abilities you're losing out on with the rifle.
Still, unless using the cannon increases the GCD from 1.5s to 2s, I don't see how you could do the same DPS with a rifle.
EDIT: TheRabbit posted while I was composing the above. I went back and double checked -- it does appear that it is NOT the weapon rating that was causing tech abilities to do more damage, but the higher amount of tech power (and other stats) on the higher rated weapon. So I would assume there would actually be NO difference cannon vs. rifle for any of these abilities.