what difference is there between nano and micro?
which one of these is more convienient and more similar to the uno?
The nano's a small Uno, micro's a small Leonardo. To me the USB arrangement on the micro/leo, which means it can be a mouse or keyboard, is a pitfa, and I personally wouldn't use one unless I needed that particular feature.
Convenient depends on what you want to do with it. Need extra hardware serial port? Need HID functionality?
Read the specs of the microcontrollers.
Micro has native USB - this means that you can use it to pretend to be a mouse/keyboard/joystick easily, and it has a hardware serial port in addition to the one you get through the native USB virtual serial port.
The downside of native USB is that if your sketch has a bad enough bug, you can hose the USB and then it becomes tricky (though not impossible) to get a new sketch onto the board.
A nano is easier to move code to from the Uno - no code changes are involved, as the nano is based on the same chip as the uno.