Alloy.com sizing help?

i'm thinking of getting this top from Alloy.com but i'm not sure what size to get. i've been reading other Answers on this site but i'm still not very sure. my measurements are: 34-34.5 bust, 26-27 waist, 35 hips. for tops i'm an S-M on forever21. the top i want to get on Alloy is made of rayon/spandex so i suppose it's stretchable if i get a S? someone please help :)