Panda and Santa each have an epic that can add 100% gold. Panda adds 125% gold via skills. Robosanta adds 25% gold for each crusader in front of him. However the equipment RoboSaint’s Christmas Cap of Coal Giving increases that bonus to 62.5% That means with the best gear Panda will add 225% gold no matter where you place him. With the best gear Robosanta will add 225% gold with just 2 crusaders in front of him. However while both Panda and Robosanta have gear that can add up to 20% DPS Robosanta has a skill that adds a further 15% DPS.
So when both are fully equipped RoboSanta sightly edges out Panda if placed behind 2 crusaders and becomes much better if placed behind 3 or even 4 crusaders. (Only possible in the Christmas event formation so far).
That being said far more players may have epic gear for Panda then for Robosanta so for them Panda may grant more gold.
Try them both out and look at your gold %. Use whichever one grants the most. But fully equipped Robosanta has a much higher potential for gold gain so he is worth getting.