For the info on what spells are blockable, check out Kristoffer O's 9/22/2007 post on the last page of
FAQ Discussion & Submission Thread and 10/03/2007 post in
Arrow Fend Thread
Similar effects? One works only once, while the other has a permanent effect. Twist Fate isn't of much use on a unit with only 10 defense, though goes up in value with ethereal or harder to hit units.
I admit, that the value of the air bless for an air nation has its flaws. But it can be powerful for a non-air nation. And for an air nation, it'll allow you to save gems and can be used before arrow fend is researched, allowing you to bypass some research, if you have a reason. For nations like MA Agartha, however, you have these limits.
1). No archers = no counterfire
2). Earth magic is pretty short ranged. You'll often have mages in forward positions if you actually want to hit something important first turn with Blade Wind (i've ocassionally carved up poorly protected Star Children casting communion slave, and that was important with forty of them). The air bless will help keep them alive while you're skipping Stoneskin in favor of a first strike.
3). You do get ironskin for protection, but lucky strikes still happen with frequency. Especially with fatigue kicking in. I've done quite well without an air bless, but I've seen plenty of situations where a 40% miss rate would have made a difference. Of course, I would be giving up something to get that air bless with Agartha's choices... but I would be giving up something with most blesses.
4). Regeneration requires another bless - or items and is hardly effective on 10 hitpoint mages. Though it's certainly useful on a lot of front-line troops.
There are situations where I would pick astral, and situations where Twist Fate is near useless.
Twist fate good? Ermor's Shadow Vestals, or T'ien Chi's (3.08?) Red Guard or Ancestor Vessel, to name a few. Where it benefits from neither air nor astral? Abysia EA's Burning Ones, with a glorious 5 or 2 defense, and 17 protection 23 hitpoints. I've outlined a situation where I prefer the air blessing- there are probably others where its handy, but I've stuck to Atlantis, Agartha, Ctis, Yomi in competitive situations, and am not very versatile.