Swatches, always, and there is a second reason beyond colour: you cannot judge drape from a photograph and drape decides whether the pattern works at all.
The same fibre in the same weight can be stiff or fluid depending on how it is woven, and a pattern designed for one falls apart in the other. Hold the swatch over your hand and see whether it falls or holds a shape. Takes two seconds and it has stopped me buying the wrong thing repeatedly.