Pierce.
The age 27 comparison is kind of unfair as it was one of Pierce's lower scoring outputs of his prime, while it's easily isaiahs best.
Also Paul was so versatile.
Yeah, Pierce had better seasons both before and after his age 27 season. 2003-04 & 2004-05 were his two worst years of his prime.
Look at Pierce's 2001-02 & 2005-06 seasons, those were his two best in terms of scoring output. Even in Pierce's best two season he only attempted 6.3 and 4.0 3PA per game. Thomas is attempting 7+ this year. Had Pierce been encouraged to shoot the three more, like in today's NBA, he probably scores a few more points per game.
The game is also played at a faster pace today than in the early 2000's. Scoring is up throughout the entire league.
Thomas is a comparable scorer, but I give the edge to Pierce.