If you are polymorph user, try the TC first, in my experience polymorphs are not practical and Moon is useless as a school. Hopefully someone with good use and experience with polymorph can give you better idea.
The Azteca Polymorphs are not bad at all. I've learned 7 Spells (including all the Azteca ones) and they've been pretty helpful in Group Play (they can be tricky in Solo fights). If you're a Group player (or what some power), you might want to try these out. These Polymorphs are by far the most powerful. They contain various Crafted spells and interesting powers.
The Storm Ptera includes Tempest so for those who do not wish to learn all the way up to Cat Burglar from Celestia for Tempest, can just train this. This Polymorph has the hightest critical rating of all Polymorphs. Now don't be fooled, you won't critical every round to cast an attack, but most times when you critical, your enemies won't Block (even the Aztecans). You'll gain Feint, Leviathan, and Catalan among other spells. The natural attack Thunder Strike deals 550 Storm damage and 10% Storm Pierce. 'Health is at 2500 so be careful when in battle.
The Life Jaguar is a step up from the Treant. Rebirth has been placed in this deck. You gain a spell 'Ritual Blade' which is like a Life Link. Critical is fair, ok power pip chance, and knows Goat Monk, Dryad, and other Life spells. 3100 Max health I believe. He has high outgoing healing so he's good in a health pinch.
The Icehorn is my favorite of all of them. He's an awesome defensive character and packed with shields and power-ups. He's deck holds Frozen Armor, Elemental and Spiritual Shield, Elemental Blade, Blade Storm, Frostbite, Stun Block and Fumori Giant (probably Ice Trap, Balefrost and Ice Blade, can't remember). His health stands at 3500. He holds a spell called Ice Breaker which deals 275 Ice damage and steals a shield.
Hope this helps. I think they're a good get, but you might think differently.
Tip: Use when you have a lot of pips stored up. In most cases, the Pip chance is not as great as your own. Plus it allows you to spam spells or immediately attack.
Spells (including all the Azteca ones) and they've been pretty helpful in Group Play (they can be tricky in Solo fights). If you're a Group player (or what some power), you might want to try these out. These Polymorphs are by far the most powerful. They contain various Crafted spells and interesting powers.