Here's a table comparing all potions you can buy from NPCs. As a rule of thumb, you should always aim for the best potion available for your vocation and level.

	Item	Type	Vocation	Level Min	Cost	Weight
	Small Health Potion	Health	Knight, Sorcerer, Druid, Paladin	0	20	1.50
	Health Potion	Health	Knight, Sorcerer, Druid, Paladin	0	45	1.50
	Strong Health Potion	Health	Knight, Paladin	50	100	2.90
	Great Health Potion	Health	Knight	80	190	3.10
	Ultimate Health Potion	Health	Knight	130	310	3.10
	Mana Potion	Mana	Knight, Sorcerer, Druid, Paladin	0	50	1.50
	Strong Mana Potion	Mana	Sorcerer, Druid, Paladin	50	80	2.90
	Great Mana Potion	Mana	Sorcerer, Druid	80	120	3.10
	Great Spirit Potion	Health + Mana	Paladin	80	190	3.10

Head to Potions for more information and to grab the Cavebot list.