I believe Mick had more impact
I am a very big Stones fan and grew up in the Elvis era. I like them both, a lot. I do think though that Elvis has had a larger impact. With impact defined as influencing the future, in this case, pop music in general. Elvis was the one, or at least a very important one, who, with many of his early covers, took black inspired blues music to a more white audience. Some came before him, but he, for various reasons, was the breakout guy. Others followed him at the time. The Stones, Beatles and countless others in the 60's, 70's and beyond, state that they were influenced very heavily on the music of the 50's., inclusive of Elvis. Also, his gyrating moves, pretty eye-opening at the time, led to many performing likewise in years to come, like Mick for instance. Or James Brown, or Michael Jackson.
I probably am a bigger Stones/Mick fan, but I do think Elvis has had the bigger musical impact. Just my opinion, not trying to convert anyone.
And I do like Elvis' gospel music better than Mick's.