I'd strongly recommend cocaine use over smoking crack, and there is a huge difference in how it reacts to your body, and long-term side effects.
Crack is disgusting, it tastes awful, and it fucks you up really bad. I've used it a number of times in my past, and while the high is amazing - All it ever did was make me feel like shit in the end. I didn't lose weight from smoking crack, it just made me lose my voice, and I'd end up sleeping for days on end.
Cocaine is a lot different, it's better for you than crack, and when I was using cocaine daily I noticed weight loss, not that I was looking for it, but it was very evident I'd dropped ten pounds or more in only a week or two.
The thing with both drugs is that when you're on cocaine or crack, you aren't going to want to eat. It isn't like drinking, where if you eat it sobers you up. Cocaine and crack just diminish any appetite you had entirely. So a lot of users end up going days and days without eating. You might as well just turn to anorexia if you're going to be using any of the two substances for weight loss.
But yeah, don't smoke crack. If anything, sniff coke.