Azel is usually better in ELO. She's easier to fit in, will always win at equal pills against a 7-powered 5 star, will win at equal pills against a 7-powered 4 star if she goes first. Furthermore, she has higher damage potential (she can do a full 13 damage in ELO if she lands 1st turn and your opponent ends up landing life gain on 2nd, 3rd, or 4th turn.) If they both connect first turn and life gain isn't going to come into play, they cause equal damage. Azel: 4 + 3 + 3 + 1 vs Karrion: 5 + 2 + 2 + 2.
Karrion's biggest asset is that almost anyone can KO with him. That can make him great with certain card combinations, assuming you can KO pretty reliably with whatever number of pills you have left after you landed him.
My take on them
7/4 vs 7/5
-7 power same difficulty to activate
-Karrion is little higher threat +
-Karrion plays much better once activated
-Azel even if played at the 3 round does respectable 7 dmg 7/7 is not bad for a 4 star used sooner she just improves on that
Often enemy packs something that makes it hard to use them early so that works for Azel, you are not that reliant on your big card. If karrion makes a hit even using half of your pills enemy will have a hard time and win is almost guaranteed.
Major difference that you pay that 1 star for karrion let you be unpredictable once you hit , azel is very predictable "optimal" play is to wait for the last round then finish the job. Azel can be improved by putting big +life and heal cards in your deck you have to do some math but using them might just be enough and catches the opponent off guard.
I'll be sticking with Azel for a while especially with carefully designed deck ,karrion is a big star hog even if you sacrifice him as bluff your remaining cards will often lack dmg to seal the deal. People go very paranoid when facing karrion 2/3 of the players overpills like a madman (6-8pill).
I say Karrion. Azel suffer the same fate as Greem. Looks good on paper, but isn't as good as you think. The min 3 is actually a very big deal, a problem which make freaks rather predictable. 7 power is kinda on the low side too, Muze is only 3* with poison min 1 which make her a much better bluff with her bonus.