ok - here's my penny's worth, from someone who hasn't had a baby yet and so whose experience is obviously limited. BUT, i have had both a spontaneous miscarriage (it happened on its own - natural, no interventions) and an induced miscarriage with misopristol (after my cervix wouldn't open during the d&c and with a couple of what i presume were prostaglandin tablets (sp?))...
the spontaneous miscarriage (which in my head i liken to a natural birth) started slowly, built up over a period of hours and then was absolutely hideous for 2 contractions, and then it was over. done.
the induced miscarriage (which i presume is similar to an induced birth) went from 0 (can't feel anything, nothing going on) to 60 (ohmigod, what the f**k is happening to me, can't move my legs, where am i, feel sick, think i'm going to pass out, ow ow ow, give me all the drugs NOW) in about 20 seconds. i had no time to build up my resistance to the pain, no time to get used to it, which is what i understand happens in a more natural birth, and the pain was excruciating for a very long time. the main thing was: i was scared. i just didn't know how long it was going to last, or how more painful it was going to be, or whether i was going to be able to cope.
don't forget: the important thing about pain is our perception of it - in the first case, it felt like period pains getting v v bad, but over a period of hours, the second was terrifying because of the speed.
sorry, that all sounded gruesome, but just thought i'd chip in.