I agree that Holyfield's prime was before age 30 (he turned 30 right before the 1st Bowe fight, which was Holyfield's 11th world title fight), however he was still quick after age 30. Look at his quick combos vs. Tyson and in the rematch with Moorer (1997, age 35). Look, neither man was slow with the punches. Size? Holyfield was definately a little bigger (height, reach, weight) and totally ripped even at 220 Lbs. Holyfield fought the big modern heavyweights. Guys who were not only big, but big, athletic, and skilled. The big guys Louis fought were pretty limited, even Max Baer, who really wasn't that big compared to the heavyweights Holyfield fought. Holyfield beat Hall of Famers Qawi x2, DeLeon, Foreman, Holmes, Tyson x2, and drew with a prime Lewis (Holyfield was 36 1/2). At age 37 he lost to Lewis but a case can be made that Holyfield won this fight... a case can also be made that Holyfield lost their first fight, a strong case actually. Holyfield also beat O. Ocasio, T. Booze, R. Parkey, and H. Tillman among others at CW. At HW he also beat the likes of Tillis, Thomas, Dokes, Rodrigues, Stewart x2, Douglas, Cooper, Bowe, Moorer, Mercer, Czyz, Bean, Ruiz, Rahman, Savarese, Oquendo, Neilsen, Botha, and others. He was about a million years old when he was robbed vs. Valuev. Holyfield beat more Lineal HW Champs than any other fighter. He beat Douglas, Foreman, Holmes, Bowe, Moorer, Tyson x2, and Rahman. He drew with Lewis. 8-4-1 (3) vs. these men... beat or drew with all of them. Louis beat 6 Lineal HW Champs (Holyfield beat 7). Louis beat Schmeling, Sharkey, Carnera, Baer, Braddock, and Walcott x2. He lost to Schmeling, Charles, and Marciano. He went 7-3 (6) vs. these men. He never fought Charles or Marciano again after losing to them. Anyway, H2H, both at their very best, I see Holyfield winning a tough fight.
I think Holyfield is hard to rate in these kind of H2H matchups with past HW greats. Remember, he fought in the era when the average HW was well over 200 pounds, whereas Louis fought in the era when many HWs were actually CWs or even LHWs by today's standards. Louis is typically remembered as more consistent and impressive champion, but would he have been just as consistently impressive if he were fighting a bigger man almost every time out? By contrast, Holyfield was widely viewed as an inconsistent champion, only an "average" HW puncher, and was even dismissed as "too small" for the division after initially moving up - yet he would've been considered a "big" HW in Louis' era. How would he have looked if he had the luxury of fighting many more 190-pound contenders like Johnny Paychek rather than 215-220 pound contenders like Adilson Rodriguez or Alex Stewart? My point is, it's hard for me to accurately assess Holyfield's attributes when compared with those of past HW greats.
But using this logic, Holyfield should've been KO'd every time he fought Bowe, Tyson, Lewis, or any big HW puncher for that matter. He was certainly in trouble against Cooper, but survived and came back to brutalize him in the remaining rounds. Louis had some similarly unexpected and rocky moments against much lesser fighters as well, just like virtually every other great fighter that ever lived. I think the question works both ways as well - what would happen if Holyfield detonated on Louis' chin? People have always looked at the Cooper fight and asked, "What if [insert fighter's name here] landed that punch instead?," but they often failed to ask whether that fighter could deal with what Holyfield brought to the table - hence the reason people mistakenly thought Tyson could beat him. We've seen Louis troubled, down, and hurt against smaller and less powerful fighters than Holyfield. After all, he was outboxed for several rounds and even wobbled late in his first fight with Conn, who didn't even weigh in over the LHW limit.
Even so, I think he would be making a grave mistake if he got drawn into trading with Louis. I don't see that scenario ending well for him.
If Holy uses movement and boxes on his toes using speed, he would get off first every time and win by stoppage mid to late rounds. If he gets static and starts brawling then it becomes a more interesting fight but i would still favor Holy by stoppage.