I'm just a nobody, but I think there's a difference between the two. Going through the motions, it feels like I've experienced the both of them at separate times, with each being fairly different. Life has a bunch of things wrong with it that greatly affects one's view with it: uncontrollable aspects, working, bills, debt, aging, etc. How you react to all of it, however, determines the motions one may end up going through and change the experience completely. A person may become apathetic to life and just not want to put up with it any longer. The roses may no longer smell as sweet, their days a repetitive drawl with the future being a stagnant chore to wade through. There's no real stress, it can just. SUCK! I wouldn't say that's depression, but definitely not liking life, possibly even to the point of CTB.
Then there's actual depression, where your feelings are spiraling or stuck in some state. Stress from life's trials and tribulations may be too heavy to endure and the feeling of despair becomes oppressive. Maybe an insurmountable sadness lingers for no reason instead. Yet, a person can still wish to push through. A loss of a family is a common reason for depression, yet it's not going to stop a person from wanting to keep going, usually. Sometimes, time does heal the wound, or maybe they do need medical help to proceed.
In the end, I think people just often feel both at once, thus the common connection between them. After all, if one CTBs, it's usually a solution to some kind of problem.