Original feeling is I don't like either card, but I'm warming up to Y'Shaarj.
Regarding Harbinger: There's no reason to get a 10 mana card in your hand before turn 10. Also not guaranteed the effect will go off. If C'Thun proves to be OP then maybe this card does see play, but I think it's a waste. I'd rather play Sylvanas, Cairne, Emperor, Justicar or the class 6 drops such has Highmane, Fire Elemental, Cabal.
My first instinct with Y'Shaarj is he won't be played unless in a very specific deck where you can predict the minion he pulls. Like the Shaman spell decks that only have Malygos and a couple other minions.
I originally compare him to Varian except he doesn't draw cards off the top, he pulls a minion from the deck, which is better. It's more predictable and guarantees something hits the board. The fact he's neutral is a plus also since not all classes care about fatigue unlike Warrior. Also Varian can leave you with 4 minions on the board, crazily overextended, and 1 clear wipes you out, this card isn't so crazy.
First thought is this card is going to be really good in a warlock deck simply because he pulls demons for free. Most demons have drawbacks to playing them, so this card would bypass the negative effects.
He's also a 10/10 which goes 2 for 1 against Giants. Not much out there can kill a 10/10. This card could potentially be the Dr. Boom in the set. Right now he looks like a pile of overpriced stats, but he has the potential to spiral out of control quickly for your opponent.
Of course BGH is a consideration right now, but I have faith he'll be changed to not be able to 1-shot Gods. Even if BGH isn't changed, this card seems powerful enough to probably see play, just like Boom was.
Regarding the Gods in this expansion, it was claimed there will be 4 Gods and all of them seem to be 10 cost. The 3 released so far are different enough to all have decks made around them. I'm liking the power levels so far and can definitely see each God in a deck somewhere.