Is there a difference between these functions? I've looked at SC2Mapster and they don't have a page for AISetStockUnitNext. Is one better than the other in certain situations, or something?

I can't give you the exact details because I'm reformatting my computer but the last argument of AISetStockUnitNext has 2 values

one is something like c_stockIdle and the other is something like c_stockAlways.  I'm going from memory here.

The idle one, which I think is the false value, doesn't tell it to not stock that item yet, but rather to build up to it only with idle production capacity.  In the testing I've done with Zerg, if you tell it to build a bunch of drones with the c_stockIdle function and nothing else is going on it will start building Drones 1 at a time, not building a new one until the last one is finished (presumably to not use up all the larva since it's only supposed to be using "idle" production capacity.)

I think there's some other difference between this and AISetStock, but I'm not entirely clear on what it is.  I stopped doing detailed testing on these because I intend to try to work without them.