I run the GRP and the V-Spec.
As a matter of fact, I've got two GRP .21s and a few V-Specs....
For me, the V-Spec is what I like. Its got power everywhere. From the bottom all the way to the top. Great engine, but because it makes great power/torque on the bottom, you have to have good throttle control.
The GRP is very good engine too. But, it lacks bottom-end. The power delivery is very linear, but its a slow curve regardless of what shoes and springs I use in the clutch when compared to the V-Spec. This can be good for those that would tend to spin the tires on the V-spec. It makes the GRP very drivable. The problem is sometimes you want that low-end grunt and its not there.
As far as longevity. I'm finding that both the GRP and V-specs are averaging about the same amount of fuel before needing a pinch or piston/sleeve. I'm not seeing one consistently outlasting the other....
So, I'd go with the V-spec....