Disposable single use warmers are cheap and convenient. They often come with adhesive to hold them in place, and they can last as long as many re-chargeable warmers. They're light and stow easily so you can carry extras. Disposables take a few minutes to begin to heat up, and they have a limited shelf life (so check the date).
Most battery-powered socks and insole warmers are rechargeable, requiring a power source or an extra set of batteries. There's the possibility the warmer can break down when you most need it. They can be turned on and off as needed, and some models offer heat settings, which is a real plus if your feet get too hot. They are heavier and bulkier than disposables and cost more.