All of those mini 7.5 watt heaters are pretty weak and I don't recommend them. There are different brands, if you read the box, some of them say that it is "preset" to maintain a temperature but they never mention what the temperature is. The hydor says that it "Safely increases water temperature by a few degrees". So in the winter time, when you leave the house unheated while out, and it turns 65-70 degrees, it might raise it by only a few degrees which is not enough. Bettas can do well in range of 76-80 degrees but anything below is not that great.
The smallest tetra heater is preset to 78 degrees, so you know what temperature it is going to maintain even when room temp gets really cold. It is 50 watts instead of 7.5 watts. You can use it in 2-10 galons, so if you ever upgrade bigger, you can keep using it. It is only a couple of dollars more.
the problem is, that you would need a 2gal tank or more. My suggestion is you wait until you upgrade your tank. I don't know how the weather is where you live, for example for me, I live in California and the room temperature keeps the water at 78 so I don't use heaters. You are really going to need a good heater for the winter when it gets cold and the 7.5 watts won't do (unless you heat your house REALLY well ALL the time). You can get a 2gal tank at walmart for 20 dollars, or look at craigslist they might have cheap used ones. If you want to be really practical like me, I keep some of mine in clear storage containers that are 5gal, they cost me 6 bucks, with gravel and decorations it doesn't look half bad. and they don't take that much space. So my suggestion is that you wait for a new bigger tank IF the temperature is good enough for now.
There is nothing wrong with a 1gallon, at least you are trying. Most bettas land in the hands of people who will never even bother to do any research and end up living only a few days or weeks.