Sinner has been a more dominant player than Alcaraz since 2024. Same number of Slams, but overall better results:
2024-26:
Sinner: 5 Slams, 2 Finals; 2 ATP Finals; 9 Masters, 20 titles overall
Alcaraz: 5 Slams, 1 Final; 0 ATP Finals; 4 Masters, 14 titles overall
Meaning, +2 ATP Finals, +5 Masters, +6 titles. I do agree that Carlos is able to reach higher highs in a given match, but Sinner has just been more consistently great. And I'm not sure if the highs we've seen from Sinner this year aren't equal to Carlos (I really look forward to their next Slam final). If Carlos's very best is still better, it is just by a hair and might depend upon context.
As for comparison to all-time greats, I think Sinner's 2024-26 run is comparable to the peak years of pretty much everyone except for the Big Three, or Laver, if we fold in his pre-Open Era career. He's been comparable to Borg in 1978-80, Lendl in 1985-87, or Pete in 1993-95 -- maybe better, as Pete was far worse on clay. Neither have had a truly incredible season yet - ala Novak's 2011 or '15, or Roger's 2004-07 run, or even Rafa in 2008/10/13, Mac in '84 or Laver in '69. But his best years are probably slightly better than Pete's, and comparable to Lendl and Borg, or even Rafa's best years.
Carlos was more consistent in 2025 than other years, but needs to back that up. He's obviously got the ability, and may even have better years in the near future, but we have to see it first. Frankly, he may be too much of a "chill guy" to have that single-pointed focus to play at his very best level for a full year. If he does, look out tour.
Comparing either to the Big Three is premature. First of all, they haven't quite reached their peak dominance level. Secondly, they haven't done it over two decades. Meaning, the Big Three's greatness is a combination of peak level (best years) and sustained dominance (career). Sincaraz is close to the former, but still a bit shy. Carlos in 2025 was also close, but not quite there. If we're going to see a peak level like the Big Three, we probably need to see it in the next few years. But the career question will take another decade to answer.