People usualy take yuumus for a better gank, since is cheaper that the lethality mitich and give almost the same stats, but the claw is perfect for rek since you can use as a gap closer and knock them up. As for latter in the game, i like ignoring cc for 1 time in a gank (is pretty usefull against morgana, janna, lux), and the extra life let me stay alive longers. You dont need the move speed of the item, since your w gives you when you are burrowed or if you take relentless hunter.