Tip size is what to look for. As Sosidge says the 1/4" tip is the key, as it will hold the heat better and give a good job. A 1,000,000 W iron with a tip the size of a sewing needle is useless for our jobs as the heat will disappear from the tip in a fraction of a second as soon as it is applied to the job.
I have a 20W 7.2v iron I run on 12v and it's perfect because it has a 6mm tip that loses virtually no heat when doing motors or battery connectors. It also has a conical tip that goes to a point, and that is perfect for servo wires, etc because as soon as you put it on anything, the heat flows straight into the job and away from the iron, preventing you overheating the job.
Weller are a fabulous brand, and it is no co-incidence that they are the iron of choice for hobbyists that do stained glass - using something approaching a 15mm tip so they don't lose heat when they put it on large lumps of lead to make the joints in a stained glass assembly! It's still only 60W bit it can cope with a huge joint when the right tip is fitted. HTH
