Depends what you value.
Kessel - 1286 GP, 413 G, 579 A, 992 PTS - Playoffs: 100 GP, 34 G, 49 A, 83 PTS
Awards: Masteron (2007)
Kessel is the NHL ironman. The only category he led the league in the regular season was GWG in 18-19.
In the playoffs, Kessel won three cups, had consecutive playoff seasons with 22 pts (10G, 12A) and 23 pts (8G, 15A).
Heatley - 869 GP, 372 G, 419 A, 791 PTS - Playoffs: 77 GP, 16 G, 47 A, 63 PTS
Awards: Calder (2002)
Heatley famously had back-to-back 50 goal seasons in 05-06 and 06-07. He led the NHL in GWG in 06-07 and 09-10.
In the playoffs, Heatley co-led the NHL with 22 pts (7G, 15A) in 2007, helping Ottawa to the finals.
If you value longevity, it's easily Kessel. He played until he was 35 and was still productive at that age. Heatley was basically finished at 31 years old. Also, Kessel broke into the league at a younger age (19 compared to Heatley who turned 21 in his rookie season).
If you value peak, then it's Heatley. He twice had 50 goal seasons and also put up 41 goal seasons in 02-03 and 07-08 and two 39 goal seasons in 08-09 and 09-10. Kessel's peak in goals was 37, which he hit twice in 11-12 and 13-14. Their assist totals at their peaks are comparable (Kessel had 58 as his career high and Heatley 55), but points is again in Heatley's favour (back-to-back 100+ point seasons with totals of 105 and 103 as well as three more 82+ pts seasons) when compared to Kessel (peak was 92, had three more 80+ point seasons).
If you value playoff success, then it's Kessel, he had more and helped win two Cups (he was a passenger in the Vegas cup win). Heatley had an excellent playoff in 2007, but he also disappeared in the finals (1G, 0A in 5 GP). As for Kessel, his finals number aren't great either (12 GP, 2 G, 6 A, 8 PTS), however, they are much better than Heatley's.