I have a Shuttle Pro, and wouldn't want to live without it now. I'm a very casual editor, just home video stuff, but having the Shuttle Pro has been great.
I've not used the Griffin PowerMate, but considered it before getting the Shuttle Pro. Main reasons I went with the Shuttle Pro and love using it now are:
1) Design: You'll wind up having your left hand (I'm a rightie) on the Shuttle Pro pretty much all the time. So, the flatter ergonomic layout of the Shuttle Pro is very conducive to this.
2) "Spin" factor: With the Shuttle Pro, it is very easy to spin the dial to go frame by frame. The design of the spin wheel is such that you can very easily do multiple rotations over a large number of frames. This wouldn't be possible with the PowerMate; you'd constantly be spinning a few frames, then having to raise your hand up and then start spinning again.
3) Buttons: There are a lot of configurable buttons on the Shuttle Pro. As opposed to a series of click/rotates/etc. you simply map your common tasks to a button. Common tasks like copy/paste/split/etc. can be mapped to buttons. I've also mapped "Save" to a key on the Shuttle Pro, since I obsessively save all changes. You really wind up using the keyboard ONLY for typing text.