I use a simple 15W Weller from many years ago, with 3 temperature settings: 270/320/370°C. The tip is slightly too sharp for SMD work (the solder doesn't cling to the front of the tip), but it works well nevertheless. I also have larger 35W & 45W irons, but they are too heavy for my tastes. One even has a fume ventilator, but that just adds to the bulk of both the iron and the cable.
Crazy as I am, I've been considering building my own thermostat-controlled iron using a JeeNode. Would allow on-screen reporting, and remote / timed shut-down. Could turn off when no light/motion is detected in the room for a while, for added safety. It's really not that different from a reflow-controller. With a KAKU or FS20 switch to control 220V, even a cheap iron would do. But hey, I've got enough pots cookin' already ;)
I'd say go for small/light. Short barrel - the closer your hand is to the soldering tip, the better (if well-insulated!) IMO.