- The kind that measure brain activity, like as Dreem or Philips SmartSleep - these, in my experience are by far the most accurate and useful devices you can buy. Dreem even has a self-run study comparing the devices to a proper sleep study setup. They are however, quite expensive.
- The kind that lays under/on top of the bed and indirectly measures heart-rate, breathing rate and body movements. These are okay as far as accuracy goes. They can't directly tell you how your sleep is but they capture enough indirect information that they can make a decent guess, at least regarding how much sleeo you get. They're differentiated from the wrist-based trackers in that they can detect any kind of body movement and they can measure your breathing rate as well. They also don't require you to have a potentially uncomfortable device strapped to your wrist.
- The kind that you wear on your wrist like Apple Watch and fitness bands - these, in my experience, are garbage. Like the under/over-bed devices, they measure your sleep indirectly but unlike those devices, they capture less useful information and require wearing a potentially uncomfortable device when you're trying to sleep.
- Smartphone apps - these, in my experience, are absolute garbage. They're not sensitive enough to pick up any useful information about your sleep state. All they can really do is tell you if you're moving or not. Maybe they can pick up snoring.
If you're interested in accurately measuring your sleep state and catching things like short periods of wakefulness or the precise amount of time it takes you to get to sleep and wake up, I can't recommend Dreem enough. I've owned one for over a year now and it's been spot-on every time. Since it's attached to your head it takes a little bit of getting used to but I don't even feel it anymore.
Since you say you're interested specifically in tracking your leg movements, I think it's pretty clear that the on/under-bed trackers are appropriate for you. Personally, I use an Emfit QS for the insane amount of data and analysis it gives you but there are a number of other products like Withings. There are also mattresses that come with the technology built-in.
I would never rely on a smartphone app or fitness band. They're better than nothing so might be good if you happen to have one on-hand but I wouldn't go out of your way to buy one for the purpose of tracking sleep.