A Nano has two more analogue inputs (A6, A7).
Compared to an Uno, the Nano has some power differences/limitiations.
The Nano runs on ~4.6volt (not 5volt) on USB supply, because of a simple diode instead of mosfet backflow protection. You might get different results from the A/D if your code relies on 5volt Aref.
You can't draw as much current for sensors etc. from the 5volt pin on external/raw supply, because of the smaller board heatsink of the regulator, and the 3.3volt pin can only supply ~30mA instead of ~150mA.
Post the details of your project if you want to know if you can replace the Uno with a Nano or ProMini.
Well, i'm using a Mega and an UNO because i'm doing a transmitter/receiver project.
For the transmitter i'm using the mega, and there are plugged a LCD display, two leds, an HC-SR04 and a nRF24L01 upgraded version.
For the receiver, smaller, i'm using the UNO, and i plugged an HC-SR04, one led and a nRF24L01.
Which are your advices?