I'm going to take "better" in a slightly different direction, and consider how the teams would compare to their peers.
The 2008 team had a +11.2 Net Rating, and a historically good defense. They were only 10th in offensive efficiency, though. I think Tatum would increase that offensive rating significantly, without hurting the defense at all. I think we'd be looking at one of the greatest teams ever in terms of point differential, and we'd probably have won 72+ games.
The 2024 was historically great offensively. But, I think Pierce would fit less well with that team than Tatum would the 2008 squad, and the improvement would be slightly less overall. They'd still be a candidate for best team of all-time, though!
I wonder what the lineups would be?
2024:
KP / Horford
Tatum
Pierce
Brown / White
Jrue
2008:
Perk
KG / Posey
Pierce / Tatum
Allen
Rondo
Head to head, 2024 wins, just due to style of play. If the 2008 team was "modernized" -- meaning KG shifts to center and adds a 3PT jumper and Pierce/Allen shoot from outside constantly, then maybe they have a shot. But as is, 2024 would just kill them from outside.