I think these are two really different cards, actually.
Mask of Memory is a card advantage engine for aggressive decks. It likes fliers, and evasion in general. It's pretty strong in my cube, and best in blue due to evasion, tapping and bounce. It is useless in defense.
Zephyr Boots is a clock-maker with smoothing built-in. It is subpar with evasion, but good with large creatures. It's best in green because it gives it "reach", breaks stallmates, uses its large beaters well, and provides smoothing. It is relevant in defense.
Note also that 2 to play 1 to equip is cheaper than 1 to play, 2 to equip.
I didn't see
skeleton key making many waves around here when it came out, so not sure that Zephyr Boots makes much more impact than that. Flying is generally better than Skulk overall, but still.
Flying is so much better than skulk that I think Zephyr Boots are way better. Skeleton Key is useless with large creatures.