We had a Magic Morning at 7 AM on a Sunday in mid-to-late June 2009, when the park was projected to be quite busy. The Magic Morning seemed VERY un-crowded (although in fairness some of that might have been because we were there during what we thought was a very busy time, so just having fewer guests was huge!). We accomplished everything we wanted to during MM. We did everything offered in Fantasyland, including the Matterhorn twice. The only omission there was Snow White, which was closed. By my camera's timestamp, the carousel itself took at least 8 minutes--if we had skipped that, we would have been in even better shape. We did make it to Space Mountain at 7:59 AM.
From the carousel around 7:45 AM:
Of course, someone here said that day wasn't very busy at all, but after the MM was over it felt awful crowded to me!
Anyway, most people who are locals will not be getting Magic Mornings, and I have heard that 70% of guests are locals. Meanwhile, MOST out-of-town visitors (not all) will be visiting the park at least 1-2 weekdays.
If your choices were an 8 AM MM on a weekday or a 7 AM on a weekend, I'd think they'd be pretty similar, or the weekday MM might even be lighter. But most people on vacations like to sleep in, so I think a 9 AM MM very well may be busier than a 7 AM one.
I reckon either will be just fine for crowds, though, so if one fits into your schedule better, go for it. (I agree with the rule to go to the first MM available to you in most cases, unless you have a reason not to. We chose Sunday instead of Saturday because we predicted it would be busier and wanted to get into the park without massive lines. Not sure if it paid off or not but it seemed to work fine.)
Have a great trip!