It depends on how much money you wish to part with and if you're going to string enough frames to justify one of the top end stringers.
I do 100-150 a month and won't spend the money on a Sensor. I have a Silent Partner Aria, which does well for me. Stringing is only one part of my business, so I have to be prudent as to where the dollars are spent.
Some of my bretheren here have converted their crank machines into electronics with the Wise Tension Head. It's a bolt on tension head that has received good reviews.
It's all in what you can afford and what you can justify.