They are totally different uses. Plan B is Emergency Back Up Birth Control
also called "Morning After" Pills for when your birth control method fails, a rubber breaks etc. It's only to be used in emergencies.
Depo Provera is a long term progesterone that stops your periods and stops ovulation. It isn't used as emergency or "Morning After" birth control. Each injection of Depo Provera lasts for 12 weeks to prevent ovulation and usually menstruation, also. It is meant to be used as long term birth control and also to control bleeding issues and uterine problems.
I've used both. Depo Provera as a treatment for endometriosis. I hated it, I gained 40 lbs, got depressed and lost my sex drive. It did stop my period, though.
I used Plan B when we had an accident. My husband, for some reason thought he was in the "back door" and he.. wasn't. When he came, I kind of freaked out, because he usually pulls out. I got some Plan B the next day. I didn't get pregnant, so it must have worked, or I wasn't ovulating. We're finished having children and I have health problems, so I was glad I was able to have it when I used it.
You take Depo Provera for months or years.
You only take ONE or two tablets of Plan B once in a great while. It isn't meant to be your main form of birth control. It is FOR emergencies.