Don't own either the Cadex or the Grex pin nailer but I have researched them extensively. They are made at the same factory with only minor modifications to differentiate them. Grex has stated that the reason they didn't choose to make the same modification Cadex has done (to fire slight head brads) is that slight head brads will not fire reliably in this design nailer. In other words, the slight heads are more susceptible to jamming to a degree that Grex felt did not meet their standards.
Having said that, many Cadex uses report no exceptional problems with slight brads. If it only increases the risk of jamming by 1 out of 1000 fires then maybe is is not a big deal (just made up that number). Personally I am leaning towards the Grex because I like the no compromise approach, they have a very extensive line of nailers and I like to standardize on brands, and of course, I like green.