Hi Olivia! Thanks! Pink Sands Resort or Palm Island for 2 adult divers, sailors, hikers in the beginning of December?
Presuming that the focus of your stay is diving I would say Palm Island because currently the dive shop based on Canouan (Pink Sands) is closed. I would contact Pink Sands to ask if they will have an on-site dive shop opened for December. If not, Palm Island is logistically a better choice. Google image a map of the area to see the layout of the islands in relation to one another.
The other private island option you may want to consider is Petit St Vincent which has the Jean-Michel Cousteau Caribbean Dive centre on island.
There is an excellent online magazine, 'Ins and Outs of St Vincent and the Grenadines' where you can compare the features of each island, www.insandoutsofsvg.com
You could also request a paper copy from one of the overseas offices of St Vincent and the Grenadines Tourism Authority in London, New York and Toronto.
The 2017 edition has a couple of pages devoted to Palm Island and Canouan.
Hope that helps!