I kinda agree with you, but, we only saw 1 fight from Ace and that was against a Yonkou. We can't really judge Ace from a single fight.
We can Judge his techs. He mainly uses fire tech (obviously). Luffy has shown the ability to protect himself from fire using haki.
Luffy speedblitz's Ace so hard. Basically Ace can't even see him.
Luffy also has a higher physical strength than Ace.
Luffy also has Kenbunshoku Haki while Ace has shown no usage of it. This means to some degree Luffy can predict Ace's moves.