Bryston 4BSST or 6BST or 7BST

I am looking at various Bryston amplifiers for my system. On the used market I can get a 4BSST for around $2200, a 6BST for around $2900 and a pair of 7BSTs for around $2900. Which amp is better sounding and which is the better deal? I will likely upgrade a few years down the road so I am looking at resale value as well.

Any difference in sound quality between 6BST vs 6BSST?

I appreciate any comments.