Although they're both '06 births, Sennecke was born in Jan and Martone in Oct. That's about a nine month difference.
Still, this is a good question. Sennecke is still growing into his body whereas Martone's physical growth is further advanced. Because Martone is more physically advanced, he was able to maximize his skills earlier than Sennecke. We really don't know how much more physical growth there is to Sennecke. What Sennecke does have going for him is his skating is better than Martone's and Sennecke's small area skills still exists as a bigger player.
I think they're both very good offensive players. Martone is younger than Sennecke by nine months. Hmmm... I wonder if Cayden Lindstrom didn't have back problems, then who would have the Ducks picked between Sennecke or Lindstrom? Lindstrom is kinda like Martone.