Tatum is better than Middleton, he actually plays defense.
When the Celtics play they way they are capable of playing, they are the better team. The real question is whether they can do that. They didn't do it often enough during the regular season or they'd be the #1 seed.
In a vacuum I agree Tatum is better, but
A. Middleton kills the Celtics
B. Game one to Middleton
Middleton 5/12, 3/4-3pt, 3/3-FT, 10 boards, 6 assists, 0 steal, 0 blocks, 2 TO in 34 minutes
Tatum 2/7, 0/1-3pt, 0/0-FT, 6 boards, 1 assist, 1 steal, 3 blocks, 0 TO in 30 minutes
Middleton is just asked to do more for the Bucks than Tatum is on average for the Celtics. I'd guess Tatum outplays Middleton (not necessarily head-to-head but stat-wise) maybe 2 times in remaining games. But Middleton will put up more pts, assists & boards in more games and more overall.