Advantages and disadvantages of Telepathy vs. Thought-speak:
The most obvious advantage of true Telepathy over the form of Thought-speak that the Animorphs and Andalites use is the consideration of distance. Thought-speak has a limited range, once the intended recipient passes beyond this; the message becomes more and more garbled, to the point where it is nearly impossible to understand.
Telepathy, on the other hand, has an almost unlimited range. And, even if the intended recipient is outside that range, they will still sense that something is being said to them. Between two telepaths, the telepathy has no limits to its range.
Then again, one of the disadvantages of telepathy is that an experienced telepath can backtrack the message to its point of origin. With thought-speak, such an action would be impossible. Telepaths are also capable of invading and controlling another person's mind, sometimes without them even knowing about it.
Generally, the more experienced the telepath, the more subtle their efforts at control will be; to the point where the victim can even believe that they are performing the actions out of their own free will, when in fact they are being completely controlled be the telepath.
But, telepathy uses a significant amount of energy. An untrained telepath attempting to reach out and control the mind of another will quickly burn themselves out, causing them to lose consciousness. An extremely well-versed telepath can even take control of a person's vital automatic functions, stopping their heartbeat, breathing, or even basic brain functioning.