Well, are you wanting a knife for the affordability? The handle material? The ergonomics?
If you're mostly after the ergonomics of the Grip, then go ahead and get the Grip. The plastic handles, while you can feel them flex, are plenty strong. There's a member on another forum who has driven over his 551 with his SUV. Three times. No damage. Chances are, if you're doing something that would break the Grip's handle, it would do other sorts of damage to a Rift, or Presidio, or Rukus, etc.
If you're simply after handle material, definitely go for the Rift. Or Presidio. Or Rukus. Or any number of other models.
As to a comparison between the Grip and the Rift - The Grip will be lighter, the blade will probably be easier to swing open and closed, and it'll be more easily replaced if necessary. The Rift will be heavier, feel beefier and stiffer (due to solid handles/liners), probably have a more solid feeling pivot area in general, and be a little slimmer in the pocket. But it will weigh more, and doesn't have a closed backspacer, if that makes a difference to you.
But without knowing exactly what you're looking for in the knife, I'd recommend the 522 Presidio Ultra. It's the price of the Grip, but with full liners, and a much more sturdy feel. Still has plastic scales, but they're plenty strong. The hollow ground blade also cuts quite nicely.