Real pH buffer is relatively inexpensive.
Baking soda is bicarbonate. pH buffer contains carbonates as well as bicarbonates, and has more "staying power" to hold the pH where it needs to be.
Good buffer targets a specific pH (8.3). It's fairly dummy-proof.
We used to use baking soda "back in the day" but it doesn't do a good job at holding the pH, and if you over-shoot, that's not good either.
Jenn