a UNO is the most documented one. Once you reach its limits, you are skilled enough to switch to the ones that may need you to change Internet found code to work, like the Nano.
Boards like the UNO also accept a vast range of add-on shield boards to expand the Arduino's capability - e.g. SD card readers, ethernet, LCD module, gamepad/switches. the UNO also allows students to replace the microcontroller (should the worst ever happen) as it is in a DIL socket.
The NANO board like Jan Cumps mentions, is better reserved for when your code is more mature / niche projects...when you have learnt the basics and maybe are plugging into your own board or using a breadboard.
