If I'm reading this right, you have or will have both on your shared plan. I just don't understand why you'd pay the ten bucks a month to add your iPad to the plan when you can just tether from your iPhone and save the ten bucks.
Have you ever relied on a tether as on-demand? It's not practical. To leave tether on constantly drains the heck out of the battery. Also, when USING the data connection it's constantly using the phone's battery, so it'll be dead in about 3-4 hours of data usage. Plus I don't want to have to pull out the phone and enable tethering anytime I want to use the data. Plus I want access to GPS, which the WiFi model cannot do. Plus $10 is the price of two days of Starbucks, it's not going to bankrupt me. Plus I just want it to work.
Was that enough reasons? It's highly impractical to suggest using the phone to tether data for more than an extremely "on occasion" instance. Having data anytime\anywhere is a huge advantage for the iPad mini... moreso than the full size, because the mini will go with me many more places than my maxiPad ever did.
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I was curious about the general demographics. I am especially curious about people who have both an iPhone data plan and iPad data plan, why did you decide to buy two data plans, do you use your iPad outside a lot?
This is by no means scientific. LOL. Most people go the cheap route over the most convenient route. It's like the people who line up with their engines running at Costco Gas to save $0.10 per gallon, yet they burn that while idling.