I've only chartered in the BVI's once, but have extensive experience in the Grenadines so I can offer some comparisons. The distances between stops will be greater than in the BVI's with more open water passages. The islands are less commercialized and less crowded, no need to rush to the next anchorage to get a ball, with the exception of Saltwhistle Bay, it always seems to be crowded there.
Costs ashore are less, we always joke the prices are the same but they're in US in the BVI's and EC in the Windwards.
There is a greater percentage of Europeans to North Americans on yachts than up north. There isn't anything like the Willie T, they have started having full moon parties on Union at the kite surfing school.
As for boat boys, you won't find any a mile off shore between Grenada and the Cays, that's mainly off the bays of St. Vincent; they do greet you at the entrance to the harbor on Bequia, but they take turns coming out to the boats. While the guys do work hard to support their families they all get along pretty well with only the occasional difference of opinion. The only place you will see boat boys on your intended route will be in Union when you stop to clear in, someone will come to offer you a ball and in the Cays where they offer all sorts of goods and services. Best guy for lobster on the beach is Desperado.
Snorkeling in the Cays is an amazing experience, the best place to see turtles is the roped off area at Baradel. The reef has good snorkeling, but is more suited for experienced people, the coral is shallow and the currents can be strong I would suggest someone stay in the dingy to offer support if someone gets into trouble. When you leave Carriacou you can always swing by Miopian (sp?) for a quick snorkel stop, be aware of the surrounding reefs and draft.