Well,.. lets start off with a quick vocabulary review...
Most spas that are being discussed and reviewed here are 'portable'...Whether they are an 8'X8' 12 person spa or a 4 person, 6' round Softub they are all portable because they are a self-contained appliance that doesnt require special building permits or plumbing.
Non portable spas are usually 'fixed' location spas like inground acrylic, gunite, or traditional coopered hot tub... they are dependant on equipment that is not self contained or a part of the water bearing appliance.
That said, smaller spas are no different than their larger counterparts in that they heat, filter, and move water. They often use the same or similar componentry, such as jetting and electric pump motors.
If youre looking for an economical entry level spa you really cant go wrong with the rotationally molded spas from Freeflow or Tuff Spa- They both have digital control systems and genuine heating elements-- (as opposed to cogeneration heating, which relies solely on the waste heat from the jet pump motor to heat the spa water.) And you can practically put them anywhere as long as its flat and level.
The shortcomings for spas of this size primarily is in layout- so wet test to see if they will fit your needs.