Mikita was clearly the better point producer. Messier doesn't beat him anywhere offensively. He might have the edge on two way play, but Mikita was not lacking in that area from what i've heard. His prime is of higher quality.
Not to mention that Mikita's trophy case is better. He doesn't have questionable award wins ( At least in terms of forwards having arguments) While all of Messier's regular season awards are rather debatable. Gretzky and Lemieux outproducing him by 12 and 24 points. They might have not received as much hart consideration due to those 2 being among their weakest Art Ross wins. Then again, the fact he also had to compete with prime Gretzky for awards negates this somewhat. Remember that when looking at his point finishes.
As for playoffs. Clear edge to Messier. In both their first decades of hockey, Mikita was 2nd in Playoff points while Messier was third. Mikita drops to 7th if we go by PPG, but everyone ahead of him only played around 20-29 games besides his own teammate (Both of them having played 82 games) and Norm Ullman. Messier drops to 4, the player ahead only having played 34 games while he played 145.
In their second-decade of hockey, Messier is 12th in playoff scoring while Mikita is 21. Going by PPG, Mikita goes up one spot while Messier drops one spot. Advantage, Messier.
Overall, I'd have to go with Mikita for his offensive dominance. His point finishes are comparable to Jagr and Crosby.
Here are the stats and info
Mikita
Awards
4 Art Ross trophies
2 Hart trophies
Hart finishes: 2, 4, 5
Points
PPG
Goals
Assists
Mark Messier
Awards
2 Harts
2 Pearsons
Conn Smythe
Hart finishes: 2, 9
Selke finishes: 8, 9, 16, 17, 28
Points
PPG
Goals
Assists