The trade-off is simpler than the internet makes it, and it is not really about money.
Book it yourself: cheaper, smaller group, usually better, and you are on your own if it goes wrong.
Book it through the ship: more expensive, bigger coach, and the ship waits for you. That is what the premium buys. Not the guide, not the bus — the fact that a delayed tour becomes the ship's problem instead of yours.
So the question is not "which is better value", it is "how bad would it be if this ran late". Walkable port, tour that stays close, plenty of slack before all-aboard: book it yourself. Two hours inland, single road back, tight turnaround: pay the ship.
Anyone been on an independent tour that cut it fine? That story is worth more than any of this advice.
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