Sort of just went through this myself.
Had iPad 2 32gb ATT since a couple weeks of release. Same for an iPhone 4s. I just got an iPad 4 32gb ATT last week off of CL (like new condition). Two reasons....first one is silly. I have a Saddleback leather case for my iPad 2 that I really like. Didn't want to give it up, and the iPad Air doesn't fit it, so I went with 'old' technology (and paid half of the price since I'm a cheap .....). Second reason I went with iPad 4 is that, like has been mentioned, I want the bigger screen to see things better (reading glasses).
I use the iPad so much more than my 4s for everything, other than for making phone calls (obviously). If VOIP apps worked better, or the 'Phone' app came on/with the iPad....I'd not have an iPhone. I'd have no problem being the 'Fred' that's constantly using a BT headset for making calls on the iPad.
My iPhone 4s will probably be in use til it's dead....still solid as a rock. Of course, unless the iP6 or iP7 just blows my socks off

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I'd say get the iPad Air