Are there any reasons to pick Nano over Micro?
The main difference between the ATmega32u4 and the ATmega328P is that the 32u4 has onboard USB. When ATmega328P are used in an Arduino they are often coupled with an FTDI USB to serial chip. The FTDI chip is about $5 so this may be where the extra cost comes in.

Having the USB chip separate is actually not a bad thing:


The Atmega328P consumes less power than the ATmega32u4
FTDI chips are common, so the drivers are on most computers, whereas the 32u4 requires an .inf file to be loaded on Windows.
The FTDI chip uses the FTDI vendor and product ID and thus can be used in large-quantity commercial projects whereas the ATmega32u4 would require purchase of a vendor id and product id from the USB group. (My conclusion after reading around the net a bit, please correct if wrong).