The best answer I can help with is this: Purchase the biggest heater your sense of aesthetics can handle. The bigger the heater (with in reason of course), the better it will be equipped it is to handle the need. Most people skimp on such things because of price or aesthetic concerns.
The slim heaters like the Hydor Slim Heater are to my way of thinking like electric blankets. They are 'dumb' units and help slow temperature swings. For the more extreme situation you are suggesting I would generally recommend a heater with a temperature adjustment and designed for a 5 gallon or so tank. It will still be small and can be hidden in the aquarium easily enough.
Lastly, check your environment to be sure it is actually suitable to keep your beta in it. If the room you are trying to keep him in is a three season room in Minnesota, you are fighting a losing battle right off. That room heated will probably plummet into the teens in a typical Minnesota winter. To keep the beta comfortable in the 70's to 80's just is going to be a challenge no matter what you would attempt (shy or a hot tub).