Got this from a walkthrough:
-If you choose Allan as your alchemist, you can have him make you the unique
Allan Brew - an armor potion 1.5 times as effective as the next best one.
Barbatos, on the other hand, will reward you with one more skill point and he
reduces the ingredient cost of potions at the max Alchemy Garden level by 3
instead of Allan's 2. Furthermore, he can make stat buffing potions for free,
though they aren't the highest quality (and cheap ingredients make for
plentiful better potions anyway). Lastly, choosing Allan forces you to give up
the Potion of Wisdom, which grants you 5 int for free. Unless you really like
Allan's personality, Barbatos is the better choice.
