LG...I think the answer to whether to buy Riviera direct depends on how you see yourself using the contract. Would it be to book "hard to reserve" villa classifications at the 11 month mark? Or would it be more of owning at a place you like and being able to book it more readily than at the 7 month window?
Riviera is a pretty big resort but I think in Disney's mind it is the "new Grand Floridian" of this era in that it is upscale with unique transportation options. And with the additions to DHS, I think that park will be a big destination park to enhance Riviera's location. So I think demand for reservations will remain strong there.
We like BLT a lot, so that's a great option as well. You really can't go wrong, though the restrictions on Riviera resale potentially could diminish the value of selling if it comes to that. Still, we don't buy DVC with a focus on selling it, so it really comes down to what you want. Of course, Riviera also offers a longer lifespan than BLT resale, so that can be an influence as well. Good luck!