I went with an M for both. Arms are slim, but not unbearably so (probably because I have twiggy arms). Chest on the corduroy one was a little tight, but off by more like half a size than a full size, if that makes sense. This is the one with the wire (should link to the product page, I think). It's along the snap line (it has metal snaps instead of buttons) on both the left and right sides as well as along the end of the sleeve cuffs. I can see how you might want to give some shirts a little extra shape like this, but it doesn't really make sense to me on this one; it's just a mid-weight checked shirt.