I don't know that I would call scrolls a full game. It has about as many cards as like 25-30% of a single magic set.
I very much recommend trying out Scrolls if you're into deep strategy games.
Hearthstone is way more complex simply due to the 9 (vs 3) classes.
3 factions yes, a fourth on the way. 3 decks? No way! Decks evolve constantly and new ones pop up, some die out - seems very healthy for such a small, young game so far :)
Just my 2 cents and experience.