Most of Boingo's hot spots in the US are
already free hot spots that anyone can connect to. No need to pay anything to Boingo for that. Go to Boingo's Web site,
Boingo Wireless Hotspot Location Map, and look at different areas to see what they have, and you'll see that you don't need Boingo for nearly all of them.
The MiFi and the Aircard are essentially the same thing, except the Aircard only works with one device (laptop) and the MiFi becomes it's own WiFi hotspot router so that you can use a laptop, phone, tablet or other WiFi enabled device at the same time. Both connect to the cell phone carrier's data network for Internet access.
I have an Aircard with Verizon, but I'm getting rid of it. My contract end-date on the Aircard just hit. It's $50 a month for 5GB of data, but I'm also grandfathered in on the unlimited data plan with my phone. So, I tether the phone to the laptop for data, unless I'm near a WiFi hot spot and I'll connect that way. The phone can also be configured as a MiFi hotspot router that can be used for multiple devices through the same connection.