Features:

1. 32-bit development board, being previously based on 8-bit AVR
2. The board features the standard 14 digital I/O ports, 6 analog channels
3. dedicated pins for I²C, SPI and UART connections.
4. Larger memory:
   1. 8 times more flash memory (256 kB)
5. 16 times more SRAM (32 kB).
6. Easy power management:
   1. The UNO R4 has a barrel jack connector and supports input voltages from 6-24 V.
7. This connector is widely popular and removes the need for additional circuitry required to step down the voltage.
8. Cross compatibility:
   1. The UNO form factor automatically makes it compatible with hundreds of existing third-party shields
      and other accessories.

Nano’s got the breadboard-ability of the Boarduino and the Mini+USB with a smaller footprint than either, so users have more breadboard space. It’s got a pin layout that works well with the Mini or the Basic Stamp (TX, RX, ATN, GND on one top, power and ground on the other).

This new version 3.0 comes with ATMEGA328 which offers more programming and data memory space. It is two layers. That makes it easier to hack and more affordable.

You end up paying less with Nano than Mini and USB combined.