It would be demo, but if you want to put that spell dmg to good use I would suggest a build like 0/40/21. It depends what you're doing. Alot of people are finished questing, finished grinding at 70, in this case ruin is way better then a felguard. Personally I balance it a little, 39/1/21 right now for half farming/half instance runs, but when I get serious about end game heroics, raids, its gonna be 40 in demo and 21 in destro (ruin = 200% crits).
I'm making my shadoweave vest in a few days, putting me at 790 shadow damage with gems. And I got a nether drop tonite so I can craft spellstrike pants. Pop the good spellthread on those, theres another 35 point jump, around 825. Add the hood and some enchants I'll be sitting near 900 shadow damage without demonology. Once I respec I should be in the ~1050 range with the demonology talents.
For pure grinding, I'm guessing 40 points in demo kinda sucks. I mean you're so close but you dont get the felguard. But take a serious look at the other talents, combined with destruction talents (up to ruin) and you see a pretty amazing raid/5 man spec. You have alot of options, sacrifice a pet, roll with imp for 20% less threat, 10% damage increase with sucubus, longer CC durations on seduce, 15% more stamina, 200% shadowbolt crits, shadowburn for finishing trash mobs, the list goes on.
In raids/5 mans the only things I would really miss from affliction are dark pact, iHoT once in a blue moon, improved life tap and soul siphon. I wouldnt even miss instant corruption much.