Becoming WSS is easier because you only have to surpass and defeat a single, lonely man, no matter how strong he is. You don't even need to be a pirate, just be strong.
To become PK you have to be an actual pirate captain and to somehow defeat or trick the four yonkous and their whole crews to reach Laugh Tale. And on top of that you need to somehow decypher the Poneglyphs after you gather them.
Becoming PK is much harder, and of course you need to be strong for that, but this doesn't necessarily mean you will be stronger than the WSS in a 1vs1.
There's no correlation of strength between the two titles. Actually, it may even be the same person who holds both titles since a WSS can become PK, and viceversa.
To become PK you have to be an actual pirate captain and to somehow defeat or trick the four yonkous and their whole crews to reach Laugh Tale. And on top of that you need to somehow decypher the Poneglyphs after you gather them.
Becoming PK is much harder, and of course you need to be strong for that, but this doesn't necessarily mean you will be stronger than the WSS in a 1vs1.
There's no correlation of strength between the two titles. Actually, it may even be the same person who holds both titles since a WSS can become PK, and viceversa.