Haywood is considerably better than Perkins offensively. Perkins is a little better at opost defense. Rebounding is about the same, with a slight edge to Haywood, I think.
Certainly Haywood is a significant upgrade over Erick Dampier, who is overpaid and getting long in the tooth.
Not sure about the offense, both are clunky in the post but effective and smart shooters. Perkins sets screens better ... Haywood's superiority over Perk is a matter of the grass being greener on the other side. Their results this year are no comparison.
Dampier 65% FG, 59% FT, 26 minutes per game, 7 pts, 8.5 reb
Haywood 56% FG, 65% FT, 33 minutes per game, 9.8 pts, 10.4 reb
Perk 61% FG, 59% FT, 29 minutes per game, 11.1 pts, 8.2 reb
Really not much to separate. Dampier is overpaid - but that's a matter of sunk cost, and not relevant to the Mavericks equation.