If you are doing the research on where you should sail, charter or bareboat in either the BVI or the Grenadines, I will try and provide some differences that might help you decide.
Maybe we can start with similarities? Both places are beautiful, both have lots of wind (depending on seasonal averages) both have lots of places to eat and provision, both places have good places to snorkel and fish, and in both places, you will always be able to see land.
In St Vincent and the Grenadines and Grenada, you will need to do more with customs. We had to go through customs in order to sail South from St Vincent, and then hit it going out (Union), and then going into Grenada (Carriacou). Each one takes around 30 minutes depending on how busy they are. Be mindful of the time and day you are clearing.
The sailboats which are available in the BVI appear to be newer and better maintained. The recent downturn of the economy has limited the amount of new boats in fleets and it seems like the older boats head South.
There are boat boys in St Vincent and the Grenadines (people who come up to your boat as you are entering a bay or just after you stop to sell you things). There are pros and cons to them, but it is a difference. We did have a guy from Trellis bay come over to Marina cay once selling tshirts and bread… The bread was good :)
There is a general feel of safety/security in the BVI, where there is a feel of lawlessness in the Grenadines to a certain extent. Both places know that any negative action towards tourists could cost them a lot of business, so I think that is a motivator against anything going too terribly wrong.
It appeared that there are less people/sailboats sailing around in the grenadines. The highest concentration of sailboats was on the South side of Grenada. The bays there are large and protected and the number of masts is quite impressive.
Coral health appears to be slightly better in the BVI.. The Indians is a pretty neat place to snorkel with lots of schools of fish, turtles, etc. With that said, we saw coral formations that you would never see in the BVI in the Grenadines and you can tell that it used to be unbelievable in the Grenadines with huge coral structures all over the place. I am no expert on coral health, but it appears that it is recovering slowly.
Fishing is different in a few ways. As far as while you are moving/trolling, you are likely to catch a barracuda in either location, and if you are headed to or from Anegada you can catch small tuna, but we haven’t experienced much else in the BVI. In the Grenadines you can hit very deep water and run into Sailfish (we caught a 90 pounder), and besides Blue fin tuna, albacore and other varieties are possible. As far as when you are moored/anchored, we caught all kinds of fish (and released them) in the BVI, and caught pretty much nothing while moored/anchored in the Grenadines.
Navigation is super easy and straight forward in the BVI. It seems like hazards are well marked or out of the way. In the Grenadines, there are Hazards all over the place and they may not be marked well. GPS with charts is required in my opinion. There are certainly hazards in the BVI as we have seen monohulls that ran aground and snapped off their keel just outside of Cane Garden Bay.
In the BVI, you are pretty much protected from large ocean swells by the density of islands, depth, and coral reef in the North East. In the Grenadines we saw 2 to 3 meter swells and one day was forecast for 4 meter swells on the Atlantic side of the islands.
Mooring balls are well maintained and the norm in the BVI, in the Grenadines it is fairly common to have some balls, but expect to anchor half the time. If you prefer balls to anchoring, you should check out the balls (if possible) prior to using. Some of them are in poor repair. They also might work differently than you are used to.. For example, you pull the rope through the top of the buoy at Mystique and The Tobago cays..
To add to the mooring comments, in the BVI you really need to hit your destination bay between 2-3pm to ensure you get a spot. We rolled into our spots sometimes at 5:30pm without an issue in the Grenadines.
Gas prices. It appears that the prices are better in Grenada than on Tortola. My cousin found a website with all the prices on it, and I will put that here when I get the link. We used ten days worth of air conditioning, water making, and motoring and it cost us 900ec. That seemed very reasonable to us.
With all of this said, there are people with much better information and knowledge than I have. We have been in the BVI a few times in December, and just once in the Grenadines in December. Your mileage may vary, but these are my experiences and perceptions.