That post should open a can of worms! Honestly, there is no one who has done a comprehensive comparison between all the tuners. However, some common sense:
Stay away from any tuner who promises you better mileage, or even implies it- "Customers claim...".
Do they inflate their HP numbers by claiming fixed percentages for "drivetrain loss"? Avoid.
Look for someone who specializes in the ST- tuning is both a science and an art and experience is the best teacher.
If you're looking for performance, ask which tuner has tuned the highest performance STs on the planet.
Take a look at dyno charts- avoid tuners who do not address boost spikes (torque spikes) at low RPM and avoid them.
Is the "tuning" hardware proprietary? Will you be able to use it on another vehicle or resell it?
Has the tuner ever had to deal with a hybrid situation? Such as different turbos- especially turbos that are using non-vacuum operated wastegates?
Does the tuner also incorporate transmission tuning?
I am in no way associated with any tuner. But, if my list makes sense, there is no choice. ZFG Racing.