This is an all day excursion.
If you go with a private tour and something happens where you run late the ship will not wait for you.
If your on a DCL excursion they will wait for you.
We have arrived late from a couple of excursions and luckily they were booked with DCL and it was no problems.
I have seen people left in port a few times.
my opinion is stay with DCL for this particular excursion.
If you were just doing a 2 hour morning tour then it would be ok.