How to know which Arduino to choose: Nano, Micro or Pro Micro? I'm puzzled at the moment with which one to choose.


Form-factor plays a significant role in my project, so I prefer the smaller boards mentioned in the title to fuller boards like the UNO, MEGA, etc.
I have an RC522 RFID read/write module and would like to use it without having to deal with level switching, as the RFID module requires 3.3V.


What can you advise? 

Thank you in advance!
I'm currently using inexpensive Arduino Nano boards running GRBL for a couple of little CNC machines.  The version I have uses an ATmega328 running at 5V and they work very well.

They may be available with 3.3V regulator instead of 5V - I don't know and I preferred to have 5V anyway.

The nano boards that I have look to be a complete Arduino: one side of the board has the ATmega and voltage regulator, the other side of the board has the USB interface chip and some LEDs.

You can purchase Chinese-made nano boards very inexpensively.  Look on eBay for sellers offering free International shipping.