Did you not see Rogers strength when he begged wb for his crew mate...
In fact, Roger became Pirate King after becoming sick and not at his peak anymore.
Which means that even if Roger was stronger than Whitebeard when younger, during God Valley for example.
After Roger was sick Whitebeard was stronger, just look how their fight ended in a draw when the Roger Pirates had Rayleigh and the Whitebeard Pirates only had teen Marco.
And Roger become the Pirate King with a foot on the coffin already.