I also have the Marina 10w heater from petsmart for my 2 gallon. It is preset, and claims to heat the tank to 78 degrees F. However, I've found that it only heats the tank to 4 - 5 degrees above room temperature. Unless you keep your home around 76 degrees F, that's not really warm enough for a betta. Remember your water temperature is not the same as the temperature in the room.
I also notice my betta hanging out around the heater more often that he visits other parts of the tank, so it must not be heating it evenly.
It does a decent job overall, but I wouldn't suggest it to anyone else. (Just from my personal experience with it.) Top Fin makes an adjustable 25w you might be interested in. It's suggested for up to 5 gallons, so it should be perfectly fine in a 2 gallon.