The 50k between Gawn and Stef is basically for his injury history. If Maxy had more runs on the board he'd be the better pick. I'd still pick him over Stef just because of his ceiling.
Not sure what makes you think Gawn has a bigger ceiling.
2015 stats:
Martin : 20 games, 7 scores over 120 (35% of games), 11 games over 100 (55%), 2 scores of 76 or under from 20 games (10%).
Gawn: 13 games, 3 games over 120 (23%) 4 games over 100 (30%), 4 scores of 75 or under from 13 games (30%).
Yes, Gawn took time to find his feet but 2 of his 3 120+ scores came against Essendon (Mckernan rucking) and Geelong (no one rucking). He only tonned up in 4 of his 13 games. Less than a third of his games. Martin cracked the ton in over half of his games. When Martin has a rare bad game, he drops into the 70s. Gawn had as many games under 80 as he did over 100, including 58, 66, 67, 75 and 76.
It might just be because I dont see much value in Gawn, and because I rate Stef massively. But I would say that Stef clearly has a higher ceiling, and is fairly consistent. Gawn is all over the shop like a mad womans breakfast.