Which goes down first, top or potting soil?
Depends on what you are trying to grow and where. Typically I would not use those together for anything. I would typically use more of garden soil and compost for ground planting. For containers definitely potting soil and maybe some compost, which won't compact as much as topsoil/garden soil.
potting soil goes down first and then topsoil.
potting soil goes down first - it usually has peat moss, perlite, vermiculite, etc. that make a much more nutritious media for your plants. top soil is usually sandy, does not hold water well, so it goes on top. you might want to put mulch on top of the top soil to hold in the moisture so you won't have to water as much.
