I think the adjustable one would be a more wise decision, I dont thiink yoou have to worry about them being "sturdy", the products made by blackhawk were designed to save and protect lives.
Now thinking about the adjustability, we don't shoot at turkeys while we are standing to often. When curled up next to a big ol' hadwood our length of pull changes, sometimes being able to shorten that butt up some makes it much more comfortable to shoot from that postion.
Hope that helps with your decision, and from what I hear the Knoxx makes a world of differeance in recoil.
Good luck!