Not to sound too simplistic, but bridging is traditionally set up between 2 APs, whereas mesh allows the ability for a non root AP to connect to another root AP if the one it connects to goes down, as well as connect to a root AP via another non-root AP. Note, the usability of the non-root AP will be affected by the hop count it takes to get back to a root AP.
3 or less is a general rule of thumb.
