I just had the same issue. I was right on the edge between medium and large. I went medium (large wasn't available in the color I wanted), and I have to wear it wide open, but it fits. I probably should have gone large though.

I can't really comment on the mushroom effect between sizes. The medium sits a bit high, meaning there is a slightly larger gap between my ears and helmet, and I can't really see the brim when I ride (manual says you should for proper fit.) The top of the helmet does not stick up higher, it's just there is less on the bottom if you can smell what I'm cookin'.

Overall, while I think it looks OK, wearing it has not made a significant difference in the wife's amorous desires.