Originally Posted by
Hornplayer
Get the other bike assembled and ride them both. I may be 1/2 inch taller than you, and ended up on the M/L. This was my first road bike and I road both at the shop. The guy watching me said he thought I would be much more comfortable on the larger frame, and swapped the stem to 90mm from the stock 100mm. It will feel like a very different position than a MTB.
Well, I already got the Medium a while back and it fits great. I just refreshed this thread asking about Giant's sizing chart. I did ride both, and the M/L felt too stretched out. Most likely I could go either way but the Medium just felt right. According to the size chart, M can go up to 5'11"... but thats just going off height. Since I have longer legs I could potentially rock the M/L as well with a shorter stem like you