I always thought, probably incorrectly, that CC employees got commissions on the sales, while BB employees do not. Therefore CC employees have a tendency to come of as being "pushy" where BB employees seem more "helpful." I find myself buying more stuff at BB, I do liken it to a religion though. CC and BB both have their fans and detractors, and there really is very little compelling reason to chose one over the other.
I don't know about commissions on sales, but CC employees did stand to pocket a substantial percentage on extended service plans. Every time I've been in a Circuit City with even half a notion of getting something more substantive than a CD, the salesperson chatted up the service plan like crazy. But then, that happens in most places that feature in-house plans like that.