After you've owned and used a hot tub for several years, you may want (or have) to get a replacement hot tub heater. Today's modern spas generally come with either electric or gas heaters, and most buyers make their preference when picking out the model they wish to purchase. Nevertheless, that doesn't mean you won't change your mind down the road, or have to face a broken down heater that needs replacing.
Older hot tubs often have inefficient, energy guzzling heaters that, over time, end up costing more to continue using than the cost of replacing it for the energy savings. You can even get a modern solar powered hot tub heater that reduces your added energy costs virtually to nil. You'll need to live in a location that gets a high percentage of unencumbered sunlight during the day, but it's worth looking into if you're worried about the high cost of heating your spa's water.
In deciding between the traditional hot tub heating methods - gas or electricity - it often comes down to which is cheaper in your area and the availability of gas. You can use propane or natural gas with many gas heaters, so that too should be factored into your decision. In terms of which is superior as a heater, you'll get good arguments from both camps. You should enlist a professional's opinion and have him detail the pros and cons of each for your specific hot tub, where you live, prices, and all the other pertinent issues.
Regardless of which style of hot tub heater you choose, it's important to note that a larger heater will heat your water faster, while a smaller one will take longer. This can be a very important factor, depending on your hot tubbing preferences. If you're a spontaneous soaker who likes to use the spa on a moment's notice, a fast-heating (larger) model will fit your lifestyle better. If you plan your soaks ahead of time and only rarely do it on the spur of the moment, then a smaller heater that gradually heats the water is a good choice.
A final tip is to pay attention to the materials used in the internal parts of any hot tub heater you consider buying. Be sure it was made with heat transfer efficiency as the main priority. Otherwise, you might inadvertently spend less for a heater that will end up costing you much more in the long run because it 'leaks' heat. You don't want to pay to heat anything but your water !