On the one hand, when FOME sent me the order for this card I almost replied “Do you mean ‘Whenever you attack with one or more creatures?’” On the other hand, my brother-in-law often has enough land to cast whatever he wants and loses to too many fairies (my wife) or a fat angel (me). If it’s turn three or close, I’m mana screwed, or I’m holding a hydra I’m going Dragons; but if I draw this late, especially against fliers, I’m going Yak.

I know basically nothing about MTG, and barely have a grasp on what land does, but Dragons seem a lot better than Yaks. Like wouldn’t Dragons basically let you empty your hand your next turn in a green deck?

I just can’t see myself ever choosing Yaks over Dragons on this one; unless most of my creatures have vigilance, blocking won;t win you the game. It doesn’t even say “Whenever a creature you control blocks a creature”, so if I have a Hundred Handed or some other “can block multiple creatures” card, it’s just one +1/+1 counter.
 
Dragons on the other hand, is very much “win more”. I mean, even if it was “Sacrifice a creature that could legally attack this turn:Add G” (i.e., you don’t care about the attacks living or dying), any ramp/convoke based deck will love this, as it can literally pay for itself if you’ve got enough tokens lying about. Hell, if and attacker survived combat and had vigilance, you could get double duty on convoke spells. It’s just an go wide payoff into huge spells.