Sam's parry stance has a significantly shorter length then Raiden does. While Sam has a higher chance to perfect parry, the player has a higher chance of going into parry stance too early, occasionally resulting in failure.
"BY THE HAMMER OF LOU RAWLS!!!"-Me during the final battle of Metal Gear Rising: Revengence
Sam also have a startup delay for the parry too. For Raiden, I can quickly go into Parry in Neutral State.. but Sam takes slightly little longer to get into parry state.
BUT one thing Sam has that has an advantage is that he can Parry Cancel from majority of his Strong attacks. So if you charge up your attack and see the enemy about to hit you, you can easily cancel into parry.
Sam is definitely harder to use than Raiden. He's very satisfying to play if you can perfect his style of attack charges and very short parry frames though. But Wolfy is gonna make you cry unless you parry a lot, that's for sure.
Frank Zappa is God. Fact. King of Heroes, do you have enough swords?!