KPPPM is slightly luminous.
KPPPM colors change at quicker intervals.
KPPPM has a visible twisty ply.
Memories is fuzzier, has brighter colors, and less obvious plies.
I've knitted with "Geranium", "Pansy" and "Morning Glory" colorways when they were in the KnitPicks 'Sock Garden' line.
I'll take KPPPM any day over Memories. the price of KPPPM is the only thing that holds me back from buying it on a regular basis.