The DROPS suspend the active ingredient in non-comedogenic oils.
The SERUM suspends the active ingredient in an emulsion of water and oil.
If you tend to dryness, the drops are your choice.
The drops should be mixed with water, or CALMING MIST (better), or REVIVE (best) and spread evenly across the skin.
For extra dry skin, a smidgen of REMEDY with REVIVE can provide an outstanding moisture barrier.
If your skin tends to the oily side, then a serum is your choice. These are designed to spread far (high slip), and leave no residual tack. You could follow with a moisturizer and be good to go.