I wouldn't say that there was noticeable difference in the durability or action of the CF or FRN models. However, the CF model was noticeably heavier and thinner. While those are minor observations, I could definitely feel a distinct difference in handling and deployment of the knife.
It's my opinion that for the design and purpose of the Stretch II there is no real benefit to CF over FRN in terms of strength and robustness. The CF just looks better. I also think that the nested steel liners of the FRN model offer just as much strength as the full steel liners of the CF one.
Also, there's a lot of talk about giving the Stretch a deeper choil. Please don't change it. It's perfect. The jimping is just right and changing it would ruin the aesthetic portion of the knife. For reference, the Para I is a perfect example of a knife that desperately needed a deeper choil.