Before buying a MiFi device or a pocket WiFi router you must check certain things to make sure you are buying a right device which would fulfill or your needs.
Locked or Unlocked
Check whether you would be using only one mobile internet service provider or in future you might change to another company. Certain mobile internet service provide a their own MiFi device router which is locked and you can use any other SIM in that device which makes portability to another service provider impossible. Unlocked MiFi routers can accept any sim card, So that you are not locked any particular service provide. Always good to Buy generic MiFi device.
3G or 4G or Both
Make sure your MiFi router supports 4G as there are nor 4G services available and sometimes it offers 4G internet at the price of 3G. So its always best to have device which supports 3G as well as 4G.
Number of devices allowed to connect
Check how many devices can be connected to the MiFi device at a time. Most of the devices now allows 10 connection. Make sure your device of choice supports at least 5 or more connections so you and your family can enjoy internet at a same time with multiple device.
Router cum Power Bank
with the increasing use of Smartphone carrying extra power bank with you is always good as Smartphone run out of batter pretty quick. So carrying a MiFi device and Power bank is a difficult task. Its better to have MiFi device which can also act as power bank.
Make sure your device support USB 2.0 so that you can connect the pen drive and can share files amongst devices connected to WiFi.