i definately think gg3 is better, again i started with fairy and it does do a job, but gg3 does a better job in my opinion, the windows shine better, and stay cleaner longer meaning the next clean is easier. Fairy must leave a residue as the dirt seems to stick to the window easier.
Not to mention the suds on the frame etc, i, myself always wipe the frame and sill off, but have noticed, especially on dark couloured u p v c, that have been cleaned by different wc in the past, that a lightening of colour takes place and in some places the brown u p v c has been worn a way down to a grey colour, by the suds being left on the frame.
yes gg3 is more expensive then fairy, but i believe you get what you pay for, but do accept that fairy can do a job.
But i bet if you could do a test on the same windows over a few cleans, the customer would be happier with the gg3 finish.