I went with Elvis. I'm not really into either one of them. They were both huge and iconic. They were both undeniably gigantic influences on modern rock music of all types. But I went with Elvis because his reach was so far that I think his influence arguably goes beyond Berry's. And he was so prolific that it is hard to make a list of his influential songs and feel like you aren't leaving something out. I also feel like he was a bit more diverse and dabbled in more genres than Berry did, but that may just be my unfamiliarity with the greater Chuck Berry catalog.
But either way, I'm not taking anything away from either of them. There's really no "wrong" answer here.