A couple of friends are building an S-21. They have experienced multiple head scratchers where the parts and plans simply don't agree. Factory support is >apparently< not of the kinder, gentler variety. Some of the choices in how the kit goes together are head scratchers for me - like the wingtips, which mount in a way that is much more challenging than necessary.
I'd bet the RV avoids most of those issues.
From an airframe standpoint, the RV is a substantially larger airplane. Probably more capable too, at least from a speed and lifting perspective. I wouldn't be surprised if the S-21 could operate out of shorter fields.