You may have to decide whether to use an electric spa heater or one powered by gas. Gas heats quite a bit faster than electricity, but requires special tubing and is more complicated to install than an electric connection.
A gas spa heater requires tubing. In most jurisdictions, only a licensed technician is allowed to tap into the natural gas lines in your home. Additionally, installing gas lines may require the use of flaring tools, a special set of tools designed to cut and manipulate copper tubing.
When it comes to electricity, the biggest problem for a spa heater is the risk of electrocution. In most cases, this is accounted for by moving the electrical parts well away from the spa, but portable or "cabinet" spas may have the electrical feeding directly into the cabinet.
Ease and Convenience
For ease and convenience, an electric hot tub heater is often the best choice. It may take a little longer for the spa to heat, but there are fewer parts to cause problems farther down the road, and you can usually do the installation yourself: simply plug it into the outlet for a clothes dryer.