Ajani, Who Is Protecting Valiantly!
So we have a 6-mana planeswalker who starts with 4 loyalty. That seems a bit low, doesn't it? That's ok, because this Ajani has two ways and not just one to gain loyalty counters.
His +2 allows you to put two +1/+1 counters on up to one target creature. This isn't a bad ability at all, but it seems rather pedestrian on sch a highly-costed planeswalker, does it not?
The +1 ability is similar to Tezzeret, Master of Metal's in that he lets you reveal cards from the top of your deck until you reveal a particular card type. In Tezzeret's case, it was artifacts. In Ajani's case, it's a creature. The rest of the revealed cards go harmlessly to the bottom of your deck in a random order. That's a decent way to get some card advantage, but again, is it worth 6 mana?
The "ultimate" -11 ability lets you put X +1/+1 counters on a target creature, where X is your life total. That creature also gains trample, but only until end of turn. While this could be a game-winning swing, 11 counters is quite a high price to pay! It's reminsicent of the avatar ability of the original Ajani Goldmane, which gave you a token with power and toughness equal to your life total. This isn't nearly as good.
As a "kitchen table" planeswalker, I feel like Valiant Protector comes up a bit short. He has decent abilities, but 6 mana is a lot to ask to be ticking up those two to get to the ultimate. He does sort of protect himself by boosting a creature. But otherwise, he may just draw you into a creature card or two before opponents take him out. I do think that there is going to be room for him in some Commander decks, though. As far as Standard playability? I don't see it happening.
The card art is really nice, though.
OK, so Ajani Unyielding also costs 6 mana to cast, and only has 4 loyalty. However, his abilities are far more powerful than his Planeswalker Deck counterpart's.
His +2 is worlds better than Valiant Protector's +1. While you're limited to the top three cards of your deck, you get to put all nonland permanent cards you reveal into your hand. Essentially, he can draw you three cards for 6 mana right away. That's fine with me. Even if you only get 1 or 2 cards out of this ability, it's well worth it.
Unyielding's -2 abiity is literally Swords to Plowshares. You exile a target creature and its controller gains life equal to its power. You can even use this ability on one of your own creatures to keep yourself alive! (Such a pro play!) It's not exciting on a 6 mana planeswalker, but it's going to be relevant by the time you get this guy in play!
Ajani's ultimate costs -9 loyalty counters and is actually pretty insane. You put 5 +1/+1 coutners on each creature you control PLUS five loyalty counters on each other planeswalkers you control. While it takes a bit to get up to this ultimate, in a "superfriends" style deck based around planeswalkers, this is pretty devastating for your opponents. Nissa, Voice of Zendikar and Gideon, Ally of Zendikar are going to have a field day with this sort of boost, especially as they create tokens themselves that get the boost!
It gets better for the Unyielding. Oath of Ajani will let you cast this guy a turn earlier! The Oath makes all of your planeswalker spells cost one less to cast, plus puts a +1/+1 counter on each creature you control when it enters the battlefield! This Ajani costing 5 mana instead of 6 makes a pretty big difference. A Green/White Planeswalker deck will be more than happy to top off its mana curve with a couple copies of this guy! Heck, this guy will be playable in Modern if you throw up enough counter-magic to protect him.
There doesn't really seem to be much contest here between the two Aether Revolt Ajani's. Ajani Unyielding is so much more powerful than Ajani, Valiant Protector, and it's not even close. However, Valiant Protector has his fans and he'll be kicking around kitchen tables and creature-heavy Commander decks for years to come.