I recommend a 25 watt Visitherm heater. A little pricey, but it WILL NOT die on you and kill your fish, if you use it right. You can use the tetra 2-15 heater. It's OK. I prefer my tanks a little warmer then 78 degrees though..
I do twice weekly 100 percent changes on my 2 gallons. Sometimes three when I'm bored.
But, you do NOT have to do this many. However, if you keep the water VERY clean, it is very likely your betta will never get sick. :)
Sooo... your choice on both! It's mostly preference!