*No reduction in damage for either

Energy Sentry
Pros:
+Rate of fire
+Level 3 "rocket" has explosive pomson effect (penetration and depletes cloak/uber)
+Projectile speed
+Range
+Sapper immunity (still becomes disabled but takes less damage from the sapper)
Build up "Overcharge" by racking up kills/assists to unleash a spurt of critical shots
Alt. Fire to activate the full "Overcharge" to take less explosive damage and unleash a fury of critical hits

Cons:
+Build/upgrade time
+Build cost
+Warm up time (like a heavy's spinup time)
+Metal consumption (needs to be reloaded consistently since it fires faster)
-Red Tape Recorder immunity (becomes disabled and deconstructs faster)
-Health
-Carry speed
Completed "Overcharge" temporarily disables sentry

The Look:
The "bullet" would look somewhat like thinned out Righteous Bison blasts
The lvl 3 "rocket" would LOOK similar to a critical pomson shot

In MVM:
Lvl 3 "rockets" slow down robots instead of deplete cloak and uber

Stat Logic:
The stats are roughly designed so that this sentry can withstand ubercharge and large pushes while making them relatively ineffective to running scouts, pyros and demoknights
(example: not good at defending from quick rushes, but good for keeping groups away from cp's and key locations)

Mini-Energy-Sentry
Pros:
+Rate of fire
+Sapper immunity
+100% chance to slow enemy
4 seconds of crits on kill
4 seconds of minicrits on assist
On destruction stuns enemies in a short radius for max 4 seconds (not if destroyed by owner)

Cons:
+Build time
+Metal consumption
-Red Tape Recorder immunity
-Carry speed
-Health
-Knockback

In MVM:
No crits/minicrits on kill/assist

Post comments for edits, constructive criticism, and further ideas!