I have the ankle and I like it. Now that they switched to neoprene, it's soooo much more comfortable (the old material was scratchy and irritating to the skin). It's red, so it stands out from either my bare skin or my black tights in winter.
I think it depends on what your person would wear the most. You may know that they don't like to wear bracelets (wrist would be out) or necklaces (fixx would be out), but they might not mind one on the shoe or ankle. Just an example.
What a great idea! I think you're a fantastic family member for caring for their safety!