Their first fight was pretty close. I think it's fair to say Charles was teetering on the edge of shotivity, and was certainly never the same guy again, afterwards. Could Marciano have beaten the Charles of 4-5 years & 30 odd tough heavyweight fights previous?
Charles by decision... Marciano was still developing in 49... He was powerful and strong but he hadn't yet learned to move his head more, and punch in head/body combination... conversely, Charles was a fresher fighter at that point in time, more resilient in the legs
Now if we are saying Marciano at his best versus Charles in 1950, I'd still favor The Rock slightly... Charles was the best light heavyweight who ever lived but he was far more vulnerable (though still excellent) as a heavyweight
I would favour Charles, but it would be with some trepidation. Marciano had his way of getting over the odds, odds he just wouldn't submit to. This was especially true if you couldn't hurt him, & I doubt Charles could really test Marciano's whiskers, or resolve, at any point. Still, Charles was swifter & threw in combination more around the time he was making what has become a fairly under-rated run as Champion at Heavy. I think the Cobra on points, relatively clear, with the Blockbuster coming on late pretty heavy.