Pierce was better.
As a rookie? No way, not even close.
I love Pierce, one of my all-time favorite players, I saw his entire career. But as a young player in the NBA he was immature and had several holes in his game, and played for a horrible team and poor coaches with teammates with poor attitudes, which contributed to a lot of bad habits that he didn't break until Doc took over.
I think the "Pierce was better" comment is in jest.
Tatum's rookie season>Pierce's rookie season
I don't even care about the stats. I watched both seasons and was excited to have both players. The main difference is that Tatum is on a better team, and thus has had a bigger impact on winning.
I don't care about all of the over-analysis by the blog boys and their advanced stats. Winning>Not winning, at least most of the time.