I like Crescendo. (not sure about cost though).
vs. Blessing : Crescendo trades +3 HP for +3 ATK over 2 extra turns. ATK is generally worth more than HP, so I think the extra delay in giving creatures the full buff helps even things out.
(Sidenote : Crescendo also makes many creatures vulnerable to Maxwell's Demon.)
Decrescendo feels weak compared to other CC.
Hm. I honestly think so too - while attack can go negative like Antimatter, the creature has to have 5 or less ATK if you want it too and efficient healing only occurs if the creature is at 3 ATK or lower (+3 HP per turn ~= 1 card. See SoG.) While I do believe it can coexist with the damage-based Shockwave, I think Decrescendo needs to be slightly more efficient to have its own use.
I've added a poll on the upgraded card's cost about this. Please vote.