Minecraft wiki entry for Best_enchantments_guide, associated text: Infinity means without Unbreaking you need 383 fewer arrows. With Unbreaking I / II / III you need on average 767/1151/1535 less arrows (1536=24 stacks). Mending means your bow will never break if you continually gain enough experience, which will likely happen. If you have an enchanting setup and a lot of iron for anvils, infinity is better, unless you can obtain large quantities of arrows with ease. Another exception would be if you got a powerful bow early game, for example from fishing. Mending would be better because you won't have a lot of iron or an enchanting table early on.