Garnett never played more than 71 games in Boston (he did that twice) and he never played more than 32.8 mpg. He suffered a major injury in year 2 and while he was still a good player, he was never great again after that injury which probably cost Boston a title or two.
Ray Allen had a healthier stint in Boston, but his prior injuries took a lot of his speed and athleticism away, such that he wasn't much more than a spot up shooter for much of his time in Boston.
When you look at it in that context, it isn't overly difficult to conclude that Irving and Hayward have far more potential to be great for far longer while members of the Celtics. And while Irving and Hayward won't ever have the overall impact and career standing as Garnett, both could eclipse Allen's overall career.
Thus, I think the correct answer is Irving and Hayward assuming they both stay with Boston for 6+ years. There is just a difference between two players not quite in their prime who stay for their prime, to two players at the tail end (KG until the injury put him out of it) or past their prime (Allen) when they joined the team.