MSOG is always preferable for me if possible. Had someone cum for me 7 times in 90 minutes last week... Yum! Plus I love hearing "that almost never happens!" :-)
I also rarely do half hour appointments unless it's with a regular, but I HAVE had one of them cum 3 times in a 30 min GfE before. I just am thrilled when someone can get as much enjoyment out of our time as possible, even if it means working hard to make them pop as many times as possible. I like working for my cum. I guess I'm a bit biased...