Since the patch 1.7 and the changes made on Warmaster, there’s one recurring question asked by most players: should you use Warmaster or Giant Slayer?

Warmaster explained

Has a 60% chance of inflicting bonus damage when attacking. Bonus damage is equal to 10% of the target Champion's MAX HP or 4% of the target's MAX HP when attacking Bosses. Bonus damage can only occur once per Skill and does not count as an extra hit.
Damage based on: [Enemy MAX HP]

The Warmaster mastery can proc only once when attacking the enemy, even if you’re hitting him 4 times.

Giant Slayer explained

Has a 30% chance of inflicting bonus damage when attacking. Bonus Damage is equal to 5% of the target's MAX HP or 2% of the Boss' MAX HP. Bonus damage can occur on each hit of a Skill, but does not counter as an extra hit.
Damage based on: [Enemy Max HP]

As explained, Giant Slayer can proc as many times as you hit the target.

Your choice

Analyse the attacks of your champion, and take into consideration the following.

Maths can be a good way to solve most of the problems. It, indeed, applies in that case.

When choosing Giant Slayer:

Number of hits % chance of having at least 1 proc % chance of not having a proc
1 30% 70%
2 51% 49%
3 65.7% 34.3%
4 75.99% 24.01%

Calculation method for “% chance of not having a proc at all: 70%^(number of hits).

Calculation method for “% chance of having at least 1 proc:  100%-(%chance of not having a proc at all).

While when using Warmaster, the odds are simple: you always have 60% chance to have at least 1 proc, and 40% chance of not having a proc at all.

Against bosses

The choice is actually quite simple, and it depends on how many hits your attacks perform. The rule is the following: if your champion hits 1 or 2 times, choose Warmaster. If it hits for 3 or 4 times, choose Giant Slayer.

Maths are good to solve this problem, and you can use the table above to prove that.

Against champions

This applies mostly to the Arena. Use Warmaster.

Since you want to have reliable damages, you can be based on the 10% given by the Warmaster, and deduce that to have at least 10% damage with Giant Slayer, you need 2 hits with proc, which based on the 30% proc chance of the mastery, gets a bit complicated (less than 10% if you have 3 hits, a bit more if you have 4 hits). It’s way less than the 60% chance offered by Warmaster.