SM vs DS corruption

So this is not about which spec is the best but which spec should get to use corruption (which spec benefits the most from using corruption).
Assuming best gear in game (Atiesh, 4/9 t3 and best offpieces, so basically Naxx BIS), all buffs on alliance side, all consumables, assuming both ZG buff and worldbuffs, warlock stats will be close to 983sp 40%crit 92% hit (lets use this as a baseline since it doesn’t matter for the numbers in the end, if both the SM/ruin lock and the DS/ruin lock has the same gear).

Your average damage shadowbolt (excluding ISB) assuming 1hit roll table and rank 10 shadowbolt is 1742 dmg per bolt (((510+((6/7)983))0,92)*1,4) The benefit of SM/ruin over DS/ruin is that SM/ruin has the nightfall talent. Nightfall talent will give you on average 0,096 extra shadowbolts per 18 seconds (assuming 100% corruption uptime) which is 1742x0,096=167,24 extra damage.

The downside to SM/ruin is that:
1) DS/ruin makes your corruption do 5% more damage than SM/ruin.
2) More shadowbolts from nightfall talent leads to higher mana cost/s which leads to more life taps which leads to less time actually spent dpsing.
3) DS/ruin makes life tap give 5% more mana than DS/ruin.

Corruption damage difference between SM/ruin and DS/ruin, assuming the stats above and rank 7 corruption is calculated like: 822 (base damage) + all your 983 spell power (corruption has a 100% coefficient) is 1805, combined with the 4/9 tier 3 set bonus (12% extra corruption damage) is 2021,6 dmg.
SM/ruin corruption therefore does 2021,6*1,10 = 2223,76 damage and DS/ruin corruption does 2021,6*1,15 = 2324,84 damage. The difference is 101,08.

Regarding the added mana cost for extra nightfall procs: With the stats mentioned above, life tap gives 420+(983*0,8)=1206,4 mana, with imp. life tap talent (included in both sm/ruin and ds/ruin builds) it is 1206,4*1,2=1447,68 mana per life tap. Life tap takes 1,5s to cast so life tap gives 1447,68*(2/3)=965,12 mana per second. The extra 0,096 shadowbolts per corruption rotation (18s) costs 0,096*372=35,712 mana for a SM/ruin specced warlock (with 2 points in cataclysm). That mana costs 35,712/965,12 = 0,037 sec to lifetap. 0,037 sec is 1/67,568 of a shadowbolt. 1/67,568 of a shadowbolt is 1742/67,568 = 25,78 dmg.

So, the damage increase from nightfall talent assuming best gear, stats, consumables and buffs is 167,24 damage per 18 seconds. However, the benefit of using DS/ruin instead is 101,08 more damage done from corruption as well as 25,78 more damage done from not having to lifetap as much as in a SM/Ruin nightfall spec. But 101,08+25,78 = 126,86 dmg, which is still less than the 167,24 damage that nightfall contributes to SM/ruin.
All this is assuming you start the fight with 0 mana (so infinite fight length), which benefits DS/ruin more so than SM/ruin. If you start the fight with full mana and never have to lifetap then you can disregard the 25,78 dmg loss from nightfall and SM/ruin benefits almost twice as much from using corruption.

TLDR: Assuming the best gear in the game, all buffs, all consumables and all buffs, as well as infinite fight length - SM/ruin still benefits more from a corruption slot than DS/ruin. However, the highest damage warlock should probably still get priority on a corruption slot, as the overall dmg gain from corruption for SM/ruin is only slightly higher than for DS/ruin.
As a conclusion, we can say that if you exclude life tap it doesn't matter what gear you have, nightfall is still twice as beneficial to corruption dmg as 5% extra dmg is. So even with BIS, wbuffs and flask etc the damage difference % is the same.