I am currently skilled 40/0/21 to have the +100% benefit for crit SBs. Is it better to Skill 41/0/20 to be able to cast UA instead? Does anyone know (or has already tried) whats better for either PvE or PvP?
Depends what you're geared for. In the long run, UA is going to outperform Ruin in pretty much any given situation. You're that far down Affliction, you're going to have specced Contagion etc, it makes no sense to me to not go for UA unless you're heavily geared towards crit.
Let's say as a rough calculation you have 1k shadow damage and 25% crit, over 18 seconds UA will do. I can't say the specifics, but let's say 2156 over 18 seconds. 2156 x1.15 (SM and Contagion), divided by 6 comes to about 413 a tick completely unmodified, 138 DPS -guaranteed-. With 1k shadow, Shadow Bolt will hit for, on average, 1643 unmodified, critting for 800 more. Chaincasting 6 bolts over 18 seconds, 2 will crit at most, granting an extra 3.2k at most. Casting UA and casting 5 bolts with the same gear, let's say that one crits and grants an extra 820 damage. 2156 (UA) + 820 = 2976 guaranteed damage. Consider that in either cycle you're going to have to renew DoTs and won't be chaincasting anyway, and it seems clear to me that UA comes out the winner.
UA in raids is great. On long boss fights, it's awesome. It does more DPS than locks with Ruin. At least from my experience (when I had it).
And of course, UA + CoT on priests/pallys in PvP is always fun.
That said, I currently have ruin. The reason is because I am doing more heroic/5-man than raids and I want the burst that ruin provides. As my guild progresses with raids, etc. I will probably pick up UA again -- but right now I want burst damage and Ruin helps to give me that.
This spreadsheet is pretty helpful with your question, too.
This is an often repeated topic on the warlock forums. Virtually every thread comes to the same conclusion: UA adds more damage than ruin at all reasonable crit levels.
However, one thing they all ignore is debuff limits. My guild tends to take a lot of shadow priests and warlocks, because we put out great dps, but after a certain point we actually do hit the 40 debuff limit and things start falling off. It is for this reason that I have ruin instead of UA, as I think it's more important to add dps on the debuff-limited encounters than it is to add an extra .5% damage when debuffs are not a limitation.
In addition, there are some other subtle advantages to going with ruin. Spell crit for warlocks is an interesting stat, because it does nothing for dots but gives massive returns on shadowbolt, as the improved shadow bolt debuff is the single largest shadow damage increasing debuff. In a raid situation, most of this increased damage is shared with other warlocks, so the advantage of a high crit rate doesn't show on the damage meters, but it is there. And if you are geared for crit, the advantages of UA over ruin are diminished.
In order for Ruin to make up the difference between getting UA or not, you have to have around 30% crit from Intellect and +Spell Crit rating at the hit cap (you need around 35% overall, but you're going to have 5/5 Devestation to get Ruin, so I left that 5% out of the listing purposefully). If you play around with a DPS spreadsheet you can see this effect at work. So, if you're looking at staying Affliction, skip Ruin and go with UA instead. If there are a ton of Affliction Warlocks that you raid along side with and you have a good +spell hit (11% or more) go with a Destruction build that gets DS as you will do comparable damage/DPS.