I wouldn't consider a Cell connection for a non-phone device. Data rates are way too pricey. Tablets are great, but they are a device more typically used in a set location with WiFi, like a home, restaurant or hotel room. For the rest of the time, I can activate a hotspot using the phone anyways.
Using a tablet that connects directly to the cell network will always be preferable IMO. It's just not $20 a month preferable to me. The built-in hotspot functionality on phones is also preferable, but that allows the carrier to "see" how much you are tethering. The app above will mask your tethering activity so the carrier thinks it's just your phone.
Non-root tether apps like Fox-Fi simply use your phone's hotspot functionality. They will not mask tethering activity. Found this out the hard way.