I chose the Fab Defense FGG-S Tactical Folding Grip over other picatinny foregrips out there, mainly because it definitely was made for the Glock in mind, as it does not only attach to the rails, but also on the lower part of the frame, and the front of the trigger guard. This provides more area to distribute the force applied of the foregrip. This minimizes the flexing of the frame when using the foregrip.
It even has a hook-like protrusion at the upper front of the grip that protects your finger from the muzzle blast. Another item I bought online, which arrived about a week later. The only problem with the grip, is that there is a bit of a gap between the FGG-S and the front area of the trigger guard. This gap allows the frame to flex a bit too much, increasing the gap between the slide and the frame, which I was afraid was going to cause malfunctions.
I decided to put some material to fill that gap between the FGG-S and the frame/trigger guard by using 3M double sided tape (used only one side, kept the backing of the other side intact). I used three layers of the 3M tape to effectively fill that gap, and they seem to work very well in minimizing the the flexing of the frame when force is applied to the FGG-S.
One thing to remember when attaching this grip, is to ensure that you don't overtighten the screw, as Glock says that tightening the screws too much of any attachment on the Picatinny rail will cause the gun to malfunction. There really is no need to tighten too much the screws for this grip, the screw is caught between teeths that keep it from moving back and forth anyway. So just use enough force to lock the screw lightly in place.