I've never ridden Marathon Plus tires. I have Marathons on my bike. I would have gotten Marathon Plus tires, but they don't come in the size I need (I commute on a folding bike).
What I can say is that in the more or less 18 months I've been riding on the Marathons, I haven't gotten a puncture. And that's commuting regularly on NYC streets, strewn with broken glass and bits of metal and who knows what. And before the Marathons, flats were a regular occurrence.