We own at Bay Club, and just got back from spending Christmas there. It was the first time we had seen (and stayed) at our unit, after its refurbishment. We love our unit - could easily live there. The size is a factor - large lanai, large space in the condo.
It is true they are next to one another. While staying at the BC, you can use the BC pool, fitness area, tennis courts, etc. OR you can use the Kohala pool as well. We were surprised this time to see more people enjoying the BC facilitites, and the Kohala property pool/bar was less crowded. The BC pool had live music two evenings - both great. You can walk everywhere within the WBR (or take the shuttle).
The big difference is in what you want - if you want to hang out at the large Hilton Waikoloa Hotel pool/lagoon, then you would want to stay at Kohala units. If you want to go wander and explore the island in the daytime, and spend time at the condo at night, then BC is fantastic!
We want to go back sooner than later... Have fun!