Lemon zest or juice
Not sure in cooking but for baking I always thought the juice would get diluted out in the wet ingredients so having pieces of zest would make the lemon flavor more pronounced.
However if you are making a glaze for a cake/muffin you would rather use the lemon juice than have pieces of zest ruining the smoothness of the glaze. Plus the glaze has so few ingredients that from experience if I add too much juice the flavor could be too overpowering whereas when adding zest to a cookie/quickbread recipe, I do by eye and don't even bother measuring.
That's what I've always though....