Re: Grenadines v BVI
I've chartered the BVI 3 times and St. Vincent and the Grenadines once two years ago (in May) out of Blue Lagoon in SVG. We made it as far south as Union Island in the Grenadines.
As for sailing conditions, your description was right on in my experience. The sails are longer and less protected in the SVG which is great for sailors. Seas and winds could build a little but nothing that a reef couldn't make comfortable. That being said if you had an inexperienced crew that might be weary of 15-20kts and 4-6 ft seas on some of the windier days, the BVI might be a better choice. Also, you probably will have to anchor everywhere in SVG. Moorings are not maintained or reliable there like they are in BVI. It's cheaper, but does require you to be comfortable selecting a spot and setting the hook. The descriptions above of petty crime were similar to the warnings we got and we had offers to "watch" the dinghy, but we always locked everything and never had any trouble.
As for the environment and culture it really depends on what you're after. As you know, the BVI can be quite crowded (no so much in the off season) and has lots of options for food, services, shopping, etc. SVG has some too but is a little more off the beaten path. You'll find fewer cruisers and more (friendly) locals. It felt more like the "real" Caribbean to me. And as one poster already commented, the Tobago Cays are spectacular - we stayed longer than we'd planned since it was so wonderful.
So, it really depends what you want out of the trip - exciting sailing and local culture or short easy days with easy mooring and lots of access to services and first-world comforts. Both have their advantages.
If I were taking inexperienced crew, I'd probably go back to the BVI to save myself work but with a crew that could lend a hand I'd be back in SVG in a heartbeat - its a terrific place. Maybe BVI this time and start making plans for SVG next time? If you like to explore, you won't regret the SVG trip for sure!