Downside of a belt is that you have a much longer way around your body, which means longer wires, and perhaps an easier chance of breaking something. A belt also implies pants that can accept a belt, which probably wouldn't work for a lot of people, at least not every day with every type/style/color of clothing.

The anklet is reasonably unobtrusive, and unless you are wearing form-fitting pants, you can keep it under your pants leg to keep it out of sight. Even if you're wearing tighter pants, shorts, or a skirt, and you don't mind the under-house-arrest tracker vibe, it's still wearable, and might make for a fun conversation topic.

Agreed that the belt is better from the standpoint of what the device is for, but I think the anklet is a bit more practical. I know I would definitely wear it more often (more or less all the time), but I could not say the same about a belt.