Jilly-
I say try it on again...if it kills you to take it off...then, that's the dress. It is a lot of money, but I don't think it is ridiculously expensive. The first dress I loved was a MS. Then I tried on several other dresses that I thought were nice too. When I went back and tried on the MS (in the color I wanted), I instantly knew it was the one. I couldn't stop looking at myself in the mirror. I was twirling and turning and basically making a fool out of myself. I didn't want to take it off. My sister gave me some advice...that at first sounded silly to me, but turned out to be oh so true.....pick the dress you love so much, you'd sleep in it.
Both dresses will be beautiful, but just from the way you talk about the MS, I think it's the "one". But definitely go and try it on again... you'll know