Tank size ratings on aquarium supplies don't mean very much and shouldn't be relied upon. They don't use any standard set of criteria, so they're really pretty arbitrary. That being said, there's a pretty wide margin for error when it comes to aquarium heaters. 2 watts per gallon is enough to keep a steady temperature up to 5-10 degrees above the ambient temperature, in my experience. Beyond that, higher wattage just means more stability, but after a certain point, you're just throwing money away (like people who think that they need 4-5 watts per gallon just to bring their temp. up to 80 degrees when their room temperature is already above 70).
The most important thing with heaters is to buy a brand that is reliable and has a solid reputation. A poorly designed or poorly built heater can explode and release toxins, killing everything in the tank pretty quickly.
