T.L.
I had the same experience when I breastfed my son. At the time I was attending La Leche League- the other moms referred to the preferred breast as "Chocolate Milk". It's no big deal, I heard the less perferred breast may have a different taste, based on something you ate (more sweet, or more bitter), or it could be like you said, slower let down, or the opposite, the let down may be too fast. You may want to pump the breast that he's not nursing on to avoid engorgement, and so your supply dosen't decrease.
Good Luck
T.