You're comparing apples to oranges - comparing a stall tactic to rush creatures.
Damage and healing are definitely related. The most common win condition is to do 100 damage before losing 100 HP. Damage and healing will both help you. Against some opponents, damage is more valuable, and against some opponents, healing is more valuable, but overall, Damage and Healing have close to equal weight.
You don't play stalls much, do you?
Having a different opinion than you doesn't mean that I have less experience with stalls than you. Of course I play stalls, they are a key part in the metagame, and not playing stalls would leave my gameplay very weak to certain strategies.
And I'd use Armagio's Gravity Pull (what I meant), not the spell card - it's still a 2-card combo, gets twice the damage, and you have a much larger HP buffer before your protection dies.
As I said before, the Angel isn't useful in a deck with Gravity Pull: your target for Gravity Pull (in this case, Armagio) makes no difference. And against many decks, the Armagio will die in one or two turns.
Ever played Dark Matter? If he has several Elite Armagios and Archangels out, he can stall very effectively. Guardian Angel may not be Archangel, but there are unupped PvP settings - and since rushes are inherently weaker in unupped settings, Angel Armagio is that much more powerful.
Dark Matter is an upgraded FG, which has very little to do with unupgraded PvP, which is what the Guardian Angel should be balanced based on. Have you tried Angel Armagio in competitive PvP? It's not a powerful combo.
I get the sense you've played against one too many firestalls.
Firestalls are the most common type of stall, so they should have more weight when balancing the game than any other stall.
Say you face a Death/Darkness opponent... what kind of CC are you going to be dealing with? Parasites probably, or Plague, or maybe Drain Life. If your opponent is Life/Light? Any CC you face is going to be Thorn Carapace. A fire deck? Guardian Angel will STILL help you, since Fire Bolt and Rage Potion aren't the only CC options - a single Guardian Angel can help your whole field survive against a Fire Shield, which is a popular option in Fire stalls, come to think of it! If you face an Air deck? You've just slowed that Owl's Eye WAAAAY down, if not halted it completely.
As I've said before, the Guardian Angel can be useful in the right situation, but those situations are too uncommon for the Guardian Angel to be worth using. I see you've thought of lots of different cards that the Guardian Angel can help you against, but those cards just aren't as popular as cards that can kill creatures in one turn. And they aren't popular enough for the Guardian Angel to be worth using.
Guardian Angel is not a rush card, its use is in stall decks - both to stabilize an offense (necessarily slow in a stall), and to bolster your defense in some deck designs. It should not be compared with other rush tactics.
Allright then, let's compare Guardian Angel to Sanctuary.
If you happen to have an Armagio in play and the opponent doesn't have enough damage to kill it in one turn, then the Guardian Angel can heal 5 health every turn for a cost of 3 quanta to play it and 1 quanta every turn, with a one turn delay.
Sanctuary can heal 4 damage every turn for a cost of 4 quanta and will also block the effects of Discord, Devourer, Black Hole, Nightmare, and Silence. There are also fewer cards that can destroy it.
Sanctuary is clearly the better choice.