SM/Ruin spec is a good start. It gives you the ability to keep Blood Pact up and it utilizes the Nightfall spec to increase your dps when you have one or more corruptions up. The downside is that it suffers when there are no available debuff slots.
DS/Ruin is used when you have reached around 500 spellpower. I don't know all the theory crafting behind it but that is what I remember. The good things with this spec is that your dps is more stable and it doesn't suffer too hard when not being able to use Corruption. The downside is that you can't have an active demon.
There are alot of useless or almost useless talents you have to take, you can ofcourse switch those around as you see fit. For example: You only need as many points in Supression to become hitcapped with your curses (with 12% hit on gear you only need 2 points in Supression)
