I have received them on both islands and in good amounts. Hard to say which is "better" as it seems to me to be more of a personal looting style. Tormenta has higher level enemies (Thrall Captains are up to level 50) and as a result I think there is a higher number of skull chests per number of kills. But unless you run laps you have to wait longer to get the number of kills. So if you are a lazy looter or a lower level it is harder to do Tormenta. Raven's Cove has ghosts the barely top level 40 and can be killed by lower levels. Also, I find most people mean South Idol when they say Raven's Cove because that is the only area with enough enemies to make it worthwhile. I rarely see anyone doing anything other than one person blundering and others vipering. This is a good loot spot for low level or unskilled looters. For looter that have weaker weapons or lack the skill points to have the power to lot alone this is a good place to work. The enemies do spawn faster there so you can get large number of kills but the rate that skulls drop is slower. You have a good chance at either island as long as you are willing to be patient and spend some time looting.