SVG Air for sure.
We had a great round-trip with them this past June. We did end up on a bit of an island hopper on the way down (BGI, Bequia, Chatham, Union, then St. Vincent), but it was a great way to see all of the anchorages from the sky, and close enough to get good pictures!
Pretty much the same on the way back - St. Vincent to Chatham to Union to Bequia to BGI. But it was still not a bad flight.
They were on time, efficient, and pleasant, and didn't lose any of our luggage...
I've never taken the ferry, not sure I would want to go that far in one on open seas...