There is multiple statements made throughout the manga of them being equal. In their last encounter, they fought for 4 days with no victor. It's not really up for debate. The two guys were equally as strong.
There is a reason why Oda created Whitebeard, and had him called Roger's equal. At the time, there was no basis of comparison for the "Pirate King" title. Oda used it a lot, but there was nothing empty speculation to go off of because Roger was dead, and Oda wasn't ready to reveal anything about him yet. So he was smart about it, and created a character who was still alive that he could use to depict just how powerful Roger was.