Death Sentry is more powerful and would most likely win. (The problem would be the context to the fight as for now the Sentry thinks that he got rid of the Void by boring him out of his body in the Sun, which is perfectly understandable ... he cured himself)
But what if Death Sentry faces the Void and realizes that he didn't get rid of him? Well, then it depends on the writer who wins in that fight.
We had more fights, where Sentry straight up annihilated the Void by beating the crap out of him in physical combat and also overall having more and better feats than the Void.
But if a writer doesn't like that approach he can make it that the Sentry flies away out of fear from the Void. Bendis would pull off something like that.
Here is the power scaling judging by the fights, feats and hyperbole statements about the Robert Reynolds:
1. Death Sentry (since it's the Sentry with an amp)
2. Sentry (since he defeated the Void, had overall much more impressive feats and on top of that he is the one, who defeated the Molecule Man)
3. Void
Now the problem is ... Sentry had a LOT of low showings. He is basically the biggest jobber in comic book history, since he lost for the sake of the plot so many times that it's not even funny anymore, but it can be explained by him having mental issues.
When it comes to the Void though, he does not break down as the Sentry does - Void does not care about anything and he goes for the kill, so he is naturally more consistent than the Sentry; not more powerful judging by fights and feats, but more consistent.