Depends on your needs.
The units you see on TV only allow limited workouts, in that they don't offer much control over the type of stimulation you want. One of the multiple channel units will allow many more options, such as doing two, four, or more muscle groups to be worked at a time. It is possible to use a reciprocating mode to enable the workout of opposing muscle groups with the same settings (i.e.. biceps Vs triceps) where one is contracted at the same time the other is relaxing. You won't get "hit" with the full strength of the contraction, but rather it will "ramp" up to the max contraction. There are many controls and options on the more advanced units that you won't find on the TV models. And, if you are experiencing a burning, not tingling, sensation, and you are using plenty of the conductive gel, your unit is probably starting to fail. This is another advantage of the more professional units, you should never experience the burning sensation. Nice thing is that with the unit I'm using, and I'm sure with many others, I have a five year warranty if there are any problems.
You can purchase a top quality unit for anywhere from $250 to $400. There may be better buys out there, I'm no professional at this. Just search around and you'll find any number. The unit I have was purchased from the Vitalityweb.com site. If you are having success with one of the TV models, then you'll be amazed at the advanced results from one of the upgraded models.