Comparing Rise of the Runelords PACG with Skull & Shackles: Some people complain RotR is too easy. It's difficult to lose a scenario, let alone have a character die. The final adventures (5 & 6) are a bit more challenging than the others, but still, many people never lose in their entire campaign.
For Skull & Shackles, Paizo upped the difficulty. It's common for people to lose multiple times in the base set scenarios alone, never mind adventure 1. So if you prefer a challenge, you're better off with Skull & Shackles. If what matters to you is a sword & sorcery theme as opposed to pirates, then by all means go for Rise of the Runelords.