GG3 is for soft water areas, gg4 for hard.
I use gg4 as in very hard water area, coming from fairy it does take a bit of getting used to as it does not give as much slide, but the end results are better.
I use both microfibre and scrim, I use mostly ergotec stuff for trad work but 95% of work is wfp now.
I used to clean frames with the same water I would wash the window and scrim or micro them after but as good as each other on that IMO, micro is better for detailing windows again IMO.
any stubborn stains would be Upvc cleaner and never once had a problem used scrim with the cleaner though dont know why didnt want to mess up the micros I guess but thats the only reason.
hope that helped a little.
Ian