I believe the tetra heater you posted is the one I have in all three of my tanks (5.5, 1.5, and a 3 gallon). I have had no problems with them and have found them to keep the temperature very reliably at 78 degrees. I have had no issues and am very satisfied with their performance so far.
I wouldn't get any of those - go for one that you can adjust yourself and not one that is pre-set. The Hydor Theo 25W heater is in that price range and would be perfect for a 3 gallon tank. You can find them at the following links :)
I think a lot of people like being in full control of the temperature and like being able to adjust it; I think this also becomes useful in treating certain illnesses if they arise and adjusting the temperature if need be. I haven't needed to do any of this yet, and so haven't bothered with getting an adjustable heater when I already have auto-adjusted ones that work perfectly well.
-Less temperature swings, from the pre-set ones I've had I know that the temperature fluctuates, sometimes up to 8 degrees if it got really cold at night. Stable temperatures are good for bettas.
-If your betta gets sick and you need to do a treatment, but need to either lower or raise the water temperature - you can't do that with a pre-set heater. It's pretty much stuck at one setting that you can't control.
-Sometimes they just may not be very effective. I've used a couple of heaters that say they work for tanks 2-5 gallons in size... So I stuck it in a 2 gallon and it won't get above 74 degrees, but then it overheats if it's in a 1 gallon.
-With the weather changes the temperature in your house changes as well, even if it's minor. In the summer you might need a heater to raise the temperature just a couple of degrees, but in the winter you might need to raise it about 5+ degrees. An adjustable heater will allow you to have the heater in your tank year-round and it will only turn on if the temperature goes below whatever it's set at.
Adjustable heaters also require less guesswork. When I used a preset one (which worked for less than a week), I found myself constantly checking the temperature. With my adjustable, I only check it every now and again.