I use a 50W Visi-therm Stealth Heater in both my 5.5 gal and 10 gal. Like Kat said, it's a very reliable heater, and will keep even a small tank at a constant temp. I've been very happy with it.
Check around at the prices by you; what I've found is that Petsmart will take all competitor prices, even from online retailers like Amazon. It gives a convenient option for those of us on a tight budget.

I'm fortunate that the Petsmart in my area takes very good care of their fish, and their Aquatics manager really knows his stuff and keeps his workers on their toes. If only all big businesses were that way......
~Kelly