A lot of it depends on your proportions, but my instinct is one should almost always to go for the smaller bike, because adjustments to make it fit larger are so much easier.

I'm 5'9" ish, and I ride a 52, and found the reach on a 54 to be long for me. The 52 is perfect, though every person I talk to wants to put me on a 54--but I have long legs and a short torso, so actually the 52 is better. My saddle is quite high, and set back towards the forward half of the saddle rails, but my stem is the stock stem.

My point is within a certain height range, there is a lot of variability, and in the end it's going to come down to your proportions, and riding the bikes and seeing what they feel like.