Smaller is a good call on a commuter. It gives you some extra standover room for all those traffic lights and stop signs. It also gives you more options for configuration, like adjusting the stem and putting a moustache handlebar on (if you're into that sort of thing on your commuter bike.)
Just be sure you don't go too small, or you can run into issues with heelstrike (if you use panniers) or back pain (from being crunched up into a too-short reach).
__________________
"I feel like my world was classier before I found cyclocross."
- Mandi M.