Nano is from 2008, and micro from 2012. Micro seems better on every parameter, in particular in having a better (I think) microcontroller, ATMega32u4, over ATMega328P.
However, all the shops I've checked out, both local and online, sell the Nano, and only some sell Micro. And Nano is always more expensive than Micro. E.g. at RS-Online 26GBP for Nano (link) and 16GBP for Micro (link).
Why do many shops only sell the inferior product?
Why is the older and less capable Nano more expensive?
Why would anyone buy the Nano over the Micro?