Which iPad to get?

I'm stuck.

I've decided that I want a tablet - but I can't decide which iPad (or even if it's an iPad - I posted a similar question to the Googleplex about the Nexus 7) to get. Apple Core folk, help me out here!

My uses:

- Reading! Textbooks, PDFs, coding/programming lesson material

- Reddit!

- General browsing

- Not a ton of content creation, but maybe in the future?

Now, I've been looking about going used, or at least refurb. One big question is how old can I go? How does the iPad 3 stack up to the 4? To the Air? I'd like a retina screen, but I don't plan on doing any gaming or anything really processor intensive.

Should I go mini? I'm not super concerned about portability - I've already got a 15" 2012 Pro in my bag, so I've lost that battle already. The Mini is a cheaper option though - how much of a pain is textbook reading/learning on those?

iPad owners - give me your insights!