So at the end of Order of the Pheonix the two dual, they seem pretty evenly matched - it turns out later Dumbledore had the Elder Wand which should've given him a massive advantage, so is Voldemort actually more powerful?? I know the real reason is "they hadn't thought of the Elder wand yet" but i'm talking inuniverse.
Maybe Dumby was pulling his punches, so to speak, because he knew it wasn't the right time for Harry to sacrifice himself and for all the yadda yadda Harry the Horcrux stuff to happen? Who knows. Does it matter? Despite some promising early entries, it's a bad story overall.
Dumbledore is more powerful but Voldemort can't be killed per say. There is a certain prophecy playing out and as its revealed at the end of the Book Order of the Phoenix, Harry is the one who has to kill Voldemort. Dumbledore defeating Voldemort wouldn't have stopped him. And as we saw later that the holder of the elder wand isn't invincible.
All Voldemort needed to do was Petrificus Totalis Lily, end Harry with an Avada between the eyes, and dominate wizard society for the next millennium because of his hor-thingies keeping him alive. So whatever his power level... he was too dumb to win.