I have a 4G mifi for work that replaced a 2200 mifi. I could have a tethering data plan instead, but I don't for 3 reasons:

1. Cellular data chews batteries. The mifi has changeable batteries, my iPhone doesn't. Using the mifi for data means longer iPhone battery life for phone calls, which is important given how much time I spend trying to work in airports, and how incredibly inconvenient it can be trying to charge up in an airport without a Delta sky club.
2. Multiple people can use the mifi simultaneously. Maybe you can do this now with tethering, but I couldn't get it to work when I tried it a few years ago.
3. You can have different networks on the devices. My iPhone is AT&T (simultaneous voice and data) and my mifi is Verizon. If one has crappy reception where I am, I can usually get emails and calendar updates using the other network.

I travel a ton, and i make my living using the phone, so these factors are critical to me.

Be aware that 4G data is outrageously expensive if you use very much of it. If I was spending my own money I would stick with the 2200 3G and just buy a replacement battery. I have two you can have, if you want them.

Jim