iPad Mini, since you already have a MacBook and iPhone - seems like the perfect medium for your needs. I own an iPad 2, but recently got the Mini since I really don't need a screen that big when I'm on the go. Now that I've gotten used to both, I severely recommend the Mini's if you want something more personal and convenient for heavy on the go purposes.
I think iPad's are great, but moreso if you're going to be stationary for the most part and need something portable and quick. For instances where you can't or don't want to bring your laptop. The iPad is not errand or discreet friendly, tbh.
It sounds like you'll always be on the move, While the iPad Air is light and convenient, the Mini might suit you more seeing you can just throw it in a small bag and always have it with you.
The Mini is good for Apps and Media. But if you plan on typing, the Air is much better.
I had the iPad 1 and it was perfect to work on. The iPad Mini wasn't. It feels easier to type on at first, but after a while your hands get cramped. I then sold the Mini to my Mum and got the Air.
Watching tv shows or movies is also much better on the Air imo. The mini cheapened the whole experience for me.
iPad 2