Have been to both within the last year. Very different cruising grounds.
The Abacos are usually relatively windy in December. The best snorkeling in the Abacos is on the barrier
reefs on the east side, but they are often churned up if the
wind is up. Temeratures, both
water and air, will be significantly cooler in the Abacos than the Grenadines. Advantages will be that most all of your sailing will be protected. Other than briefly when passing a cut, or if you go around "the Whale" from the northern Abacos to the southern or visa versa, you will be sailing in the lee.
The
trade winds will be blowing in the Grenadines in December as well and you will have open ocean sailing between islands. Distances are greater so you will spend more of your time traveling. It is more likely the snorkeling will be better as there are lots of good spots that are on the lee sides; I'm thinking of Bequia, Mustique, Union, Mayreau and Cariacou. The
Tobago Keys are hit or miss depending on the
wind. However, keep in mind the recent
hurricane damage. It is likely that Union and Caricou will have many businesses still trying to get going again. There may be issues with wrecks in the
anchorages, etc.