Giga drain is a Special move which let you has some hip drained, Leaf blade is a psychical move with higher power that can let the critical hit rate more
However, Leaf storm is the best option if your Serperior has a HA, and it’s extremely strong.
Seems You has also planned the move Coil, which boost your Atk, Def, Acc by 1 stage, Ill say that leaf blade is the better, which again, if it’s a HA one, give it leafstrom.
Hope this help!
(sorry for not reading the question carefully, I forget about the coil)