I have iPad 2, 3 and Air. All with 64 GB (128GB for the Air) and a reconditioned iPad Mini (1G). Probably no reconditioned iPad Airs available yet. I routinely buy reconditioned electronics when available.
All have cell phone capability which I have never used. IMHO, in most circumstances, you will get the most bang for the buck from more memory. If you carry a cell phone, you can use it as a hotspot for the iPhone as gdarwin suggested. Verify that your cell phone can act as a hotspot and that your carrier allows tethering. ATT charges a $10 monthly fee for using your cell phone as a hot spot. Don't know about other carriers.
On the other hand, if you have wireless service with ATT, only $10 per month to add an iPad with LTE service. No contact, start and stop service any time for a month. Not pushing ATT, just citing my experience. I recently switched to Consumer Cellular, saving about $65 per month, including paying off our two iPhones' ETF. But we are both very light users.
Unlocking refers to removing the code that restricts your phone to one carrier. All iPads come unlocked, so that is not a concern.
i bought an Android 7" tablet (EV3A TEGRA Note 7) last October, excellent specs for the price: $200 at Newegg. If I were starting out from scratch, I'd buy an Android tablet rather than an iPad. Same essential capability and much cheaper. Most iPad apps also come in an Android flavor. Takes a couple of days to adjust to Android, but not too much different from iOS.
Which you buy is a matter of personal preference. Both are good operating systems with good hardware.