I'm not sure but I do have experience in making a deck like that...
I usually have one Hagatha's Scheme and one Volcano together in a deck, two Hagatha's Scheme seems a bit too much for me but people still seem to add two more on their deck, so one of each on your deck should do the trick.
If you don't have space for any one of them, I think Volcano is more superior if you are really sure that you can keep board clearing it and it can shut down large minion like Lich King, otherwise Hagatha's scheme can stop against late-game threats or suddenly there are more number of minions on enemy board such as hoard of beasts from Hunter or hoard of demons from Warlocks, something like that.
EDIT: Volcano is always better. But Hagatha's Scheme shouldn't be underestimated either.
Volcano in isolation is better but I'd say scheme is better against current decks in wild. You just need to Mulligan correctly. Big warlock , resurrection priest, jade druid won't worry about volcano but scheme can clear a huge board. Which can set up a turn to shudder. Against aggro you may only need the scheme to be at 3 or 4 for a board wipe anyway. So personally I'd run scheme at the moment. Currently sitting at around legend 300 in wild on EU
Until turn 10, there will be no board, whichVolcano can't deal with - it goes through divine shields, it work just after you draw it, it clears big and small stuff together, it synergizes with your Acolyte of Pain.
How much turns after turn 10 you need to survive, to pull shuderwock combo?
The main issue with Volcano is that while yes, it is an excellent topdeck, it also locks you out of comboing off due to overload, where Scheme does not. If you're on the ropes and pull off a clutch Volcano to save your butt, your opponent just refills next turn, threatening lethal again, and you have to somehow try and survive another turn before you can get your combo going. That can mean Volcano essentially just causes you to die one turn later, unless you pull Healing Rain to withstand the damage. And then you can still die the turn you play out your Shudder. Scheme does not have that issue.
I run Shudderwock in both of my Shaman decks right now but neither of them is your typical OTK. One is a value Elemental Shaman that runs Shudder just as a refill, and there I expect the game to go slow and go long, so I run both 2x Volcano and 2x Scheme, with Storm Chasers to fetch them. Works very well against Res Priests and Even Shamans, as you can just repeatedly wipe their boards no worry and exhaust them out of resources. The other Shudderwock deck I run is "OTK" Jade, that just swarms the board with massive Jades through Shudder and locks the opponent out with 2x Loatheb trigger at the same time (Loatheb + Zola). Since Jades are very board-centric, I only run 2x Volcano there in case I fall behind. It really depends on how long you expect your deck to go. I'd personally run both in the classic OTK Shudder, but it's not a deck I've ever actually played personally, as I dislike playing "solved" decks.
I like playing 2 schemes and 1 Volcano in Wild control shaman.
Scheme is just GG against aggro if you keep it in the starting hand, but can also deal with huge minions from big priest, big warlock, N'zoth, etc. Volcano can't clear a full board of medium-big sized minions, but is a much better top deck than scheme and can act as single target removal, acolyte of pain trigger, or board clear, it's very versatile.
Scheme is also better than Volcano with spell damage totem, this is significant.