The difference is that Intelligent falls under Legendary, and Sentient is under Extraordinary.
That’s a key to noticing the differences between the 2.
The item is sentient and possesses its own psyche and personality, including mental and social attributes. It also has the ability to communicate audibly or telepathically. The item description will detail any attribute scores as well as communication limits (such as the ability to only communicate with the wielder).
The item becomes either self-aware or capable of basic human reasoning. It has no inherent bond with its creator, and is treated like any other NPC. It gets its own turn and array of actions. The item gains no mental or social attributes, only the ability to think. At the GM’s discretion, it may also gain a particular mode of movement, such as walking, climbing, flying, or swimming.
For sentient it tells you right there what “treated like any other NPC” means: it gets its own turn and array of actions.
What it means by no mental or social attributes, is, it doesn’t get to put attribute points into that essentially, it just is able to think on its own.
Intelligent is Sentient PLUS has own psyche and personality, etc etc etc. Intelligent includes sentient in it.
Sentient is treated like an NPC in that it gets its own actions (Major, move, minor) to do things. It may or may not have actual speed, and might only move around via someone else moving it though. That’s up to the GM of course.