The Pro Mini 5V is essentially a Uno in a tiny footprint, with a SM AT328, and no serial to USB converter (where a Nano is also Uno compatible includes the serial to USB converter, so is larger). The Pro Mini is very convenient for building in projects as it's so small, and you can develop on a Uno, and then just still the code in a Pro Mini.
Just one word of warning, you need to be wary of the Pro Mini 3.3V - rather bizarrely the 328 will only run at a lower speed at 3.3V, so it runs at 8MHz instead of 16MHz for the 5V version.