Is this shirt too small?

http://imgur.com/a/BdyCQ

I have been wearing mediums my whole life, but I'm starting to think I can find better fitting clothes. I'm wearing a GAP Standard Fit Small in the picture. I think it fits better than the medium, except possibly in the chest and shoulders, where it may be too tight.

I tried on a small at a J Crew once. It fit like the GAP one in the album, but I ended up getting medium because that's what I'm used to. If this GAP is a good fit, I'll order either a standard or slim small from J Crew (no in-person store in my area). Does anybody know if a slim would be smaller than a standard in the chest? It was my understanding that it's just slimmer in the torso and the same in the chest, but I'm not certain.

I'm 5'8", 150 pounds, 39" chest, 31" waist.