So let's look at the moves.
Grass Type, 90 BP, 100 Acc. High Crits can be clutch, and it is a really solid STAB option for more Physical Grass types.
Grass Type, 130 BP, 90 Acc. Good, right? Well, it lowers SpA by two. This is good for a Pokemon with a mixed attacking stat, if it uses Leaf Storm and other Physical moves (that means that Leaf Storm is less of a hinderance dropping SpA.
Leaf Blade is the better option usually, unless you have a way to negate the SpA drops, like White Herb/Calm Mind/Nasty Plot etc.
It also depends on Serperior's Ability. If it has Overgrow, then Leaf Blade is better. If it has Contrary, It's hidden ability, then that means that it instead doubles Serperior's SpA instead. If your Serperior has Contrary, then Leaf Storm is the much better option.
Hope I helped, good luck training your Serperior! :D