I agree that if you're unsure then the iPad Air is the safest bet, but it depends a bit on what other devices you have. If you don't have an iPhone and don't intend to buy one, or if you have a lightweight laptop that you often tote around then the iPad Mini might be more justifyable in terms of filling the gap.
I bought my first 64Gb iPad in 2010 and moved to the 3rd gen model in 2012. I have always found 64Gb to be plenty but I am looking forward to moving to an iPad Air this year as my current model is heavy enough to cause fatigue when held for a reasonable period of time.
I'm a developer by trade. The device would be for mostly testing plus casual usage only. Probably web browsing plus a small amount of apps with lots install/uninstall. I'm hoping to gain an appreciation of Apple User's UX and expectation.