That depends, in part, on how many you need to make and how much you can afford to pay for parts. If you are making thousands of units you can increase profit by looking for the least expensive processor that will do the job. Similarly, if you are making devices that won't sell if the price is above $5 there is no way you can afford to put a $20 part in it.
If the board is inside a sealed case then you don't have to worry about appearances. However, the Arduino UNO isn't much more than an ATmega328p chip and a power supply. If you design your own board you can use a $2-3 chip instead of a $20 board.