I think I trust NutFix for where I bike. But I'm also thinking about the visual effect of a rear wheel with no U-lock around it. If a thief sees the wheel inside the U-lock, he might keep walking. If he sees only the frame locked, he might look more closely. Then he'd notice the NutFix and either keep walking or perhaps try to defeat it. I suppose the latter scenario is an argument for getting the larger lock. And perhaps for continuing to use two U-locks, since an unlocked front wheel is also inviting. Just mulling whether it's all worth the extra weight and cost.