Strength of the grid isn't the only factor to consider when choosing a geogrid. You should also look at the stiffness of the grid. A grid that is "floppy", like a geotextile, will only work if it is placed in tension. The usual way for a fabric to go into tension is for the underlying layer to fail. Of course you don't want this. The grid that will contain the gravel particles in a strong interlocked position will more effectively spread the load from above on the underlying layer of clay, and I'm assuming it's a pretty low strength clay, or you wouldn't need the geogrid.
Look for the grid that will maintain the best angle at the joints when you apply a force between the joints, and deflects the least at the point where the force is applied. This is the one that will allow the best interlocking. There are some geogrids on the market that should only be used as shopping baskets, they are fine when a force is applied along their length, but are easy to pull apart at the joints.
Be careful of the suppliers of geogrids, they are worse than used car salesmen. There are some studies around that have been done by various Universities on the different performances of the geogrids, and these are the best to base your assessments on. There is an enormous amount of ignorance in the market about geogrids, and strength is only one of the characteristics to consider.