I personally like the ERT-300 for its consistency.
It make its own vibrations to determine the tension of the racket(you need to use the wheel that comes with the ERT-300 to translate dynamic tension to actual tension).
http://www.tennis-warehouse.com/descpage-ERT300.html
Check out this video.
This talks about the ERT-300 and the racket tune app(I have never used racket tune).
http://www.youtube.com/watch?v=FcXvDWI3EwU
Dynamic Tension:
Stringbed stiffness (measured in DT - Dynamic Tension) is the major parameter that determines the playing characteristics of a tennis racquet. This value represents the force which is necessary to push down the stringbed for 1 cm in a contact area about the size of a tennis ball. The DT value is dependent on a number of parameters: string pattern, string, tension, stringer, time point of measurement and so on.
Hope this information helps.