Greater Heal is based on Magery alone.
Either way, the only way someone is healing a pet faster with magery than vet is if the vet is lagging really bad (or has serious random number issues), and the mage is a FC 2 caster with a macro set up to cast multiple back to back heals.
It's also possible that there were multiple mages casting at the same time.
Greater Heal Formula (straight from UO.com):
Target is healed by 40% (caster's Magery) + (1-10)
(41-50 pts at 100 magery, 49-58 at 120 magery)
Vet formulae (from Stratics)
Min_Healed (before factoring in pet's max HP)=(lore / 5) + (vet / 5) + 3
For a GM/GM this is 43; For a 120/120 it is 51
Max_Healed (before factoring in per's max HP)=(lore / 5) + (vet / 2) + 10
For a GM/GM this is 80; for a 120/120 it is 94 (24+60+10)
Amount Healed=random number between Min_Healed & Max_Healed, + (pet's max HP / 100)
For a 900 HP Greater dragon, this would be
GM/GM = 43 to 80, +9, or a range of 52-89
120/120 = 51 to 94, +9, or a range of 60-103
So, a 120 mage can heal 49-58, while a 120 Vet/120 Lore tamer can heal a pet for about the same amount, MINIMUM - with a chance for about twice as much as the mage.
A smart tamer with magery will do both - and if the Greater heal hits about right, the bandage and spell will hit at the same time, making it appear that it was entirely from the spell, if you miss the bandage sound effect in the spell sound effect.