You are right that BBB has build-in analog pins so there is no need to add extra ADC chip. Other than that, the price difference also comes from the fact that OSPi is in much higher demand than OSBo.
Both versions have pinouts for all available GPIO pins. You can take a look at the user manual for details:
For ospi: http://rayshobby.net/docs/ospi14_manual.pdf
For osbo: http://rayshobby.net/docs/osbo11_manual.pdf