I chose 3. The Cohort is under player control 90% of the time, 5% I recommend alternate actions that the cohort could have thought of, 4% I speak as the cohort providing the information that I would give any character of the cohort's background, and 1% of the time I tell the player he's nuts and the cohort has a much saner idea.
Cohorts won't betray PCs but PCs can betray cohorts, driving them away. Dead cohorts may not come back from the dead if they died a particularly heroic death, were fairly religious, and the PC doesn't explicitly need them. Cohorts from foreign lands or of different races may also have a different vantage point on societal issues.
Cohorts will also have their own baseline for "good idea/bad idea" that I try to make clear early on. One of the key points tends to be that cohorts may make decisions that favor their Leader more than the party, such as abandoning the party by teleporting a mortally injured leader to safety. They generally aren't selfish but IMO the cohort will give their Leader the same priority someone would give their siblings.