I bought this a size up from what I'd normally get and regret it. I was hoping for a looser fit because I was expecting the top to be a softer, less stiff fabric, but I was unfortunately wrong. The is a stiff, compression type fabric that is meant to be tight and will only look weird and loose in areas like the underarm if you size up. With that in mind, I would recommend going with your normal size. The other issue I had was the bust area. I'm a 34D/DD (sometimes a 36) and I found this extremely tight on my bust. I would say that's almost a good thing, since you don't really want them moving around when you're working out, but I'd honestly rather it not be so snug and I can just wear a good sports bra if I really need them held down. My experience with nylon is that it is usually a bit softer, but I guess I was wrong.
I will say that the color (Epic Green) was gorgeous and very true to the picture.
I think the biggest thing for me with this top is really the way it feels. I was expecting a softer fabric that was more comfortable, but I got a stiff, very generic workout material top, so I will be returning this. Oh well.