Jager, Eheim and Hydor are top-quality heaters. For a 10g tank a 50W will do. Don't forget to get a cheap thermometer to monitor your water temperature.
With one Betta in a 10g setup, I highly recommend you cycle with the fish in the tank. fishless cycling (with ammonia) creates much more nitrifying bacteria than a Betta needs. A fish-in cycle happens automatically; you'd have to take steps to prevent it. Just keep an eye on your ammonia; the levels should stay <0.25ppm. But a 50% water change every week should keep it safe. Use Prime (2-drops/gal) with water changes or when ammonia is present to keep it detoxified.