Shadow mastery/ruin ftw. +10% shadow damage, +5% to crit, crits do +100% more bonus damage, after a crit the next 5 sources of shadow dmg do 20% more... you are talking serious fucking damage , esp with +% crit and +damage gear
Ruin is nice, but for PvE (Non-raiding) and PvP Soul Link is amazing. It truely is overpowered. Sure, it can be dispelled in PvP (Half of the people don't even know how to do it anyway), but against classes that can't dispel (everyone that isn't a Shaman or Priest...), it's literally like having 6 shields.
The crits from Ruin may seem very awesome, but they're not reliable. You can two shot some people, but a lot of the times you're going to be too intent on casting Shadow Bolt and Soul Fire to realize a Rogue behind you just stealthed and will soon come up and two shot you. Soul Link is awesome because all you have to do is hop around, cast a few dots on people and hop around some more.