Both cases offer fantastic protection, I have used both in the time I have owned my iPhone 4S.
They both have pros and cons, neither is perfect but they do what they say on the tin.
My personal preference is the Gumdrop case but it is a very close run thing. I have reviewed the Griffin case and gave it full marks but I find this one, the Gumdrop more usable.
1. The screen area is larger so you don't tend to bump up against the edge of the screen protector rim so much. It makes navigating the screen so much better.
2. The home button is more recessed on the Gumdrop whereas on the Griffin it protrudes and it is easy to accidentally press it in your pocket.
3. The Gumdrop has less bulk and fits into your pocket better.
4. The quality of the silicon is better, it feels harder and less likely to open the port covers by accident.
5. The camera lens is not covered making it easier to take those 'just happening' photos. It is also protected by a raised plastic flange.
The Griffin outscores this Gumdrop in that although it is not entirely waterproof,it has a lens protecting flap and has a slightly better built in screen protector. The Gumdrop screen protector may show what is commonly called oil slicking where the screen protector is not entirely flush with the phone screen. The Griffin screen protector sits away from the screen so there is a bit of noticeable give when you touch the screen.
Either or, it doesn't really matter if you want best protection for your investment however there are always compromises to be made. For me this one is more pocketable and doesn't detract from the usability of the phone in any way. Also, a point to note is that the Gumdrop for the iPad is exceptionally good, except for the screen protector, its what I use.