Kindle fire or ipad mini: don't even bother looking at the kindle. It's effectively just a shop front for amazon's content, and not well supported by developers so don't expect much in the way of apps. (It's basically an android tablet, which is already a poorly supported beast, and amazon have their own app market with terms that most devs seriously don't like!)
Full sized ipad or mini? That's a tough one. Ipad 2 or mini is easy: mini. You get the same hardware in a much more portable package. iPad 3/4 or mini? That really comes down to how you'll use it.
I have an ipad 3 and a mini ("benefit" of being a dev, I have to buy them for testing.. downside of being a dev, I have to pay for them all
). I use the mini most of the time, and I bought a nice leather case for it because I carry it with me a lot of the time. It's a *LOT* more convenient, it'll easily slip into a bag or a big pocket. And it's a lot better to handle too - it just feels so much lighter and smaller that it's more of a pleasure to hold. If it was as fast as the ipad 3 and had a retina screen I seriously think I'd *never* use the ipad 3.
However, there's the flip side. The ipad 3/4 screen is simply stunning. For photography work, a screen this good is an amazing thing. It's not like working on a computer screen any more, it's like working on a 10" glossy print. For photo editing I'd definitely go for the "big and chunky" model (seems crazy saying that about something as svelte as an ipad, but compared to the mini it seriously is!)
So I reckon it comes down to how much editing you'll do on it, and how much you'll carry it around.