I know the original miniseries portrayed them as more or less equal, but under Bendis at least would you say the Void was the more powerful of the two thanks to a more stable mental state?

Assuming for a moment the two are in potential equally as powerful as the other in every way under Bendis Bob was highly unstable. Some days he could not get out of bed or leave his house. During battle a minor thing could set him off and send him running. Whenever Bob tapped into too much power he either started to lose control or started to become the Void.

By contrast, the Void was only in control of his powers. He never seemed to be out of control of them the same way Sentry often was when reaching the higher levels of power. When the Void wanted to stop he stopped. Everything he did was something he wanted to do. Granted, he was evil so he would appear to be out of control or did not care if his unleashed powers hurt anyone, but he still came across as being in control.

So...was the Void in a sense more powerful than the Sentry or at least more effective at using those powers than Bob thanks to his mental stability?

I wouldnt say more powerful, i'd say he was just the more dominant personality, bob used everything he had to keep it at bay.

The way I see it during the Jenkins era they were two separate entities and juding by all of their fist fights Sentry was the more powerful one. Later on during the Bendis era, where Void was constantly taking over and enslaved the Sentry he was wielding more power. So you could say that Sentry is the more powerful one, if he is not in the typical Bendis-retard-mode.

Agreed with above

Actually....there is the notion that the Sentry is a hard light construct projected by Robert Reynolds subconscious. The Void is the true, real version of Robert Reynolds. The Sentry is just an ultra powered projection of Robert's conscience, a super powered Jiminy Cricket, if you will.