The U.S. Department of Energy (DOE) publishes a formula for estimating energy consumption: wattage multiplied by hours used per day, divided by 1,000, equals an appliance’s daily kilowatt-hour consumption. The cost of using the appliance can be calculated by multiplying that daily consumption by the price your local utility charges for a kilowatt of electricity -- that figure should be printed on your bills. If the iron will be used frequently its wattage can noticeably effect the amount of electricity you consume; i.e., a larger wattage iron will cost more to run than a smaller one. The DOE gives 1,000 to 1,800 watts as the range in which household clothes irons are typically manufactured.
Using the DOE’s figures as the range in which most irons on the market are manufactured, it's possible to make general recommendations concerning wattage. Irons rated at 1,000 watts or less aren't likely to be serviceable on heavier materials, including denim. Ironing times will be laboriously long, and the features available are likely to be minimal. Irons rated between 1,200 and 1,400 watts are typical, and are likely to be entirely adequate for most household duties. Irons rated at between 1,400 and 1,800 watts will press clothes speedily and typically have many onboard features, but require more care in use so as to not damage fabric.
Cost is always a factor and -- as with most things in life -- you're likely to get what you pay for when purchasing an iron. A higher wattage equates to a quicker preheat and a higher potential maximum working temperature. It also equates to a bigger heating element and heavier construction, so a lower wattage iron is likely to be cheaper than a higher wattage model.
A higher-end model, with a greater wattage, will typically have more features. Specifically useful is a wider variety of heat settings that make it safe to iron different materials.
Steam irons deliver an on-demand burst of steam through the ironing plate and directly onto the garment being ironed. This is intended to deliver professional quality results by simulating the steam finishing presses used in laundries. Only higher wattage irons are effective as steam irons, because the heating elements must be powerful enough to boil water and maintain it in a pressurized state. The larger wattage means a higher purchase cost, as does the need for additional features such as the water reservoir and the conduits to the steam vents.