I do feel roger is stronger by a smidge
but it was close enough to not defeat him handily in the fights they had .
There is a chance we don't know if either of them ever won their duel because Oda decided not to kill one's hype. ( My bet is Garp would lose the hype here ).
In my honest opinion, the edge always went to Roger but they never finished the duel because of and etc etc...
Their personalities also don't scream "murderer" or bonafide killer but I think there are at least two examples that can indicate Roger would kill you if you push his buttons.
First, Squard's crew, if I recall his experience with Roger was one of those drastic ones. The second one would be the difference between WB and Roger where WB preferred to keep his family safe to the point where he was afraid when he heard there is one Marine ship nearby lol, whereas Roger had constant hunger for battle and was stated to murder/off you if you do something to one of his crew members.
Roger vs Garp was more than likely a sparring match, such as his encounter with Whitebeard. I believe if Garp critically injured or even worse kill one of Roger's crewmates Roger would not be as respectful during a fight as he probably was whenever they met.
Garp intended to capture him but I doubt he ever went as far as to try and kill Roger.
+ My ultimate trump card, Kaido.
He fought all of the OGs and his Mt. Rushmore had Roger, Xebec and etc...
No Big Mom, no Garp, no Shik and no Sengoku. He summed it up by saying that Haki > everything and that's why Roger was at the peak of the world. No Garp was mentioned.
tl;dr Roger > Garp.