My thoughts on the proposed cards (I will not beforehand that I primarily play Limited and Cube - the later of which will be heavily influencing my opinions):
Consuming Contract vs Double Down
Double Down is a pretty useless card in singleton formats. Easy as pie choice. Not sure if Consuming Contract offers enough upside for a card that reads "Suspend 4 - You lose the game".
Blood in the Watering Can vs Mass Mummification
Few cards in Cube are sweeter than engine cards. Black has so many good cards that already incidentally make you lose life that stapling a free Raise Dead onto those effects is sweet. The thought of turning my fetchlands into free Raise Deads is also sweet. It combos well with Pox like effects (Pox is a sweet Cube archetype). It also works great in races or creature on creature matchups. Every time your opponent hurts you, you're getting a dude back. Over time there is no way for your opponent to profitably trade with you. Plus Black has tons of 187 kill dudes which makes this card even sweeter. I just hope the card costs at most 3 CMC, even if it has to be BBB.
Soulfeaster's Rising vs Revenge of Necromancy
These both kind of suck. Getting a discard deck to work in Cube is though and eventually they just run out of cards. The other card just seems like too much work. I guess I'll vote for the later because I know a friend of mine who loves discard decks and the later card could go into one of his casual decks?
Eldritch Rites vs Demonic Bargain
The way Rites is worded it seems like you have to pay the mana costs of black spells you give flashback to. If so, meh. Saccing dudes, even nontoken dudes, is no problem with Black's recursive dudes and a sac outlet is always nice but I want more upside. As for the other card is it like Mana Bloom with a mana cost of XB? How one gets the counters on there is kind of important. Should be super neat with proliferate cards

I imagine this would best fit in some B/X or X/b control deck that finds a turn in which it can tap out and then draw cards for a while before have to deal with (Bounce, Sac effects, Exile, etc.) the demon token. Kind of like a cheaper Aeon Chronicler (but is vulnerable to counters, disenchants, and gives them a dude).
I feel like pretty much any of these options will get costed to oblivion, but some more so than others.
Yeah this is even more important than what the card that wins does to be honest. I don't care how sweet the card is, I'm not playing it if it costs a billion mana. Too many games I'll be dead and/or it will be stranded in my hand for it to matter
