Wow. This is so much more detailed information than I ever expected from this post. It sure doesn't seem like it matters much at all, really. I'll probably stick to the AP just because I don't meleve MP can reduce a target below 0 MR, and thus AP will be more effective against creeps.
I usually play Soraka, so I see absolutely no use for it. And in pub games, people really don't stack magic resist. You would be lucky to see someone even buy an armor. Most of the time, people just stack damage items. That reduces the value of MP even more.
I stack CD reduction like a mad man today and got Soraka's Wish (500+2*AP heal for the entire map) down to 36sec cooldown from 60 seconds. Good fun.