How to know which Arduino to choose: Nano, Micro or Pro Micro?
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.