The 328PB is backwards-compatible enough with the 328P that you can substitute a (cheaper) 328PB and run 328P firmware on it without any problems. The PB has extra IO pins (A6 and A7 become digital-capable, and some of the power pins become IO), an extra UART, and extra I2C and SPI peripheral, two extra times. But none of that will come into play if you don't try to turn them on.
Even the hardware is sort-of pin-compatible (you end up connecting Vcc to an pin of the PB that is GPIO, but since it defaults to input and 328P software won't set it otherwise, that doesn't actually cause problems.)
Also, the 328P is "Not Recommended for new designs" (but the 328PB is not available in DIP 
)