The Ergon GS1 is marketed as a marathon/MTB racing grip. It's pretty similar to the GP1, but with geometric edges. The performance difference is that Ergon uses a different rubber compound that is meant to damp vibrations. Whether or not you'd notice the difference...unlikely. I'd say get this grip if it jives aesthetically better to you, or if the GP1 is out of stock.
If you've got numb fingies but just can't get behind the big ol paddle grip, check out the GA3.