Of the two choices, I would go for the 3% damage set bonus since you won't always be at the damage cap when soloing. Recovery and more hit points would be balanced, at least in my playstyle from spamming transfusion and transference for the debuffing along with the attendant self-replenishment.
Triumphant Defenders Forever
Psylenz FF/Psi, ArticQuark Storm/Rad, Symon BarSisyphus Bots/psn, Max VanSydow Thugs/Dk, Cyclone Symon Bots/stm, Blue Loki Ice/Cd, Widow 46526
HelinCarnate:OMG it is so terrible. I have the option to take 3 more powers but no additional slots. Boo F'ing hoo.
i 2nd going for more damage
@Injenius~Virtue
Stonefather - 50 Stone/WP/Soul brute
Sable Affliction - Earth/Dark/Fire Dom
Wild Cipher - Beast/Time MM
Definitely expedient reinforcement. Call to arms has better bonuses, but if you're only doing 4 slots then you want the greater enhancement values that expedient reinforcement will give you.
Thank you for the input, everyone. I'll stick with the Expedient Reinforcement I have in there. 
I'm looking over the build of my main, and this is the only power that I'm not sure whether to keep the current slotting or change. So, for Voltaic Sentinel, should I keep Expedient Reinforcement (4 slots) with small mez resist, 3% damage and 6,25% recharge or move to the Call to Arms set and get 2% Recovery, 1.5% hit points and again 6,25% recharge?
The switch would mean losing some slotted damage in the power - but it's mostly used solo anyway, when I have the 30% damage bonus from Vigilance, not to mention that I should achieve perma-AM again with the planned build (taking alpha-slots into account) adding to the overall damage output. I don't need the recovery and hit points, but I feel they might be a better option than a meager mez resist and a damage bonus overshadowed by Accelerate Metabolism.
So, opinions? The four slots are all this power is getting, by the way.