Both are great packs with some differences. The BH is a little bigger, a little lighter and has the outside pockets.
The big difference with the LB is that it is compatible with the NICE frame. The NICE frame is designed to carry heavy loads and be used with different platforms. As others have mentioned, with the NICE frame you can use the LB for day trips or multi day bivy hunts and then switch out the bag for the 6500 for the serious stuff.
I went with the LB for the versatility and load carrying. I can do bivy hunts with it and carry a load of boned out meat (along with my gear) if successful.
