I believe each artists' styles were perfectly suited for the time in which they are/were drawing. I can't see Yamato's more serious style fitting with the story the way Mato's did, and at the same time I can't imagine Mato being able to handle the FR/LG and BF series like Yamato is. Maybe it's just because I'm used to each's artists' style for each period, but I feel as though the writing has evolved and that the art has been part (while not a cause) of that transformation.
But in the end, if I had to choose one artist over the other, I would say Yamato. I'm not a big fan of chibiness, and his more realistic and serious tone is much more suiting. I will never get over how much I hated how adults looked during Mato's run. Outside of integral characters like Oak, Giovanni, the E4, etc., regular civilians all looked like garden gnomes. Yea, that'st just part of the chibi style, but I didn't like it. So Yamato gets my vote.