The advantages of the Pro Mini:
- Lower cost when you have multiple uses for the USB-TTL serial adapter or if you are using an ISP programmer to program the board and have no need for the USB-TTL serial chip.
- If you destroy the Pro Mini, you can still use your USB-TTL serial adapter, or vice versa.
- Smaller size.
- I believe it has lower power usage since you don't need to power the USB-TTL serial adapter chip when it's not being used.
Amy0406:
the mini looks much harder to set up
I wouldn't go so far as "much harder". The trick is to get a USB-TTL serial adapter that has the "FTDI header" pinout, which is the pinout of header on the Pro Mini. If you get that and make sure the header on the Pro Mini corresponds to the gender on the adapter then you only need to make sure you plug the adapter in the right way. At that point there is essentially no difference between Pro Mini and Nano. In fact there is now an extra complication of the Nano in that you need to figure out which bootloader your Nano has on it make the correct Tools > Processor menu selection accordingly. And of course if you have the new bootloader then you need to make sure you have Arduino AVR Boards 1.6.21 installed. There is no such confusion with Pro Mini because they all have the same bootloader. Of course that complexity is far from insurmountable but certainly some people have been having a surprisingly hard time dealing with it.
That said, I do think the Nano is the best board for a beginner unless you are going to be using multiple boards and are on a very tight budget, in which case the Pro Mini takes the lead.