I use both. The #40 is better but smells worse than the GC. The #40 is more of a one step on lightly dirty surfaces although I always wipe down with a wet rag first. The GC definately requires a cleaning of the surface prior to application either with a damp rag or with a solution like #39.
I like the GC. It's not that bad nor is it slimy like Armor All. As far as shine goes, just swipe with a MF after application and you get a nice mat finish.
Overall #40 is better, but don't hesiate to try the GC even if its just for sh!ts and giggles.