A couple of issues some people have with the public ferries is the crowding of some of the boats, the interior condition and the diesel smell. If you are on one of the nicer/newer ferries it can be a big difference versus one of the older ones with old airplane seats bolted in.

We have used GB 3 times and never left more than a couple of minutes off the scheduled time.

I have used both and only had a problem one time and that was with the public ferry (no reservations as it is first come/first served). A large number of people were heading to SXM for a gathering of some sorts and we just about did not get on and it was the last ferry.

Either way will usually get you there and back but I personally like the convenience of GB since we stay in Simpson Bay. The idea of a reservation is not bad either, just in case.