Charizard w Air Slash

AS vs. FS:
Same base power
Same energy gain
Different cooldown time: AS 1.2 seconds, FS 1.1 seconds

FS is clearly superior, despite AS is also ranked as A.

The cases when AS is arguably superior to FS due to type advantages:
Fighting
Dragon (unlikely unless you have Dragon Claw)
Fire
Water (unlikely, water hits Charizard with super effective damage)

AS only has an advantage over Heracross. But if it isn't in your area, it's strictly inferior to FS/OH.

Use it against Exeggutor works fine. I have my own 100 IV Air slash Fire Blast.

FS has a slightly shorter cool down, and slightly greater energy and damage yields

FS and AS have similar type coverage, how every in some cases (e.g. Against dragonite, water or fire types, or other zards) AS is favorable over FS

I have the same moveset (and don't have a FS/OH one). I'm waiting till gym rework to determine if I max it out, which I probably will. My reasoning is that it's an interesting dual type w STAB - there are other good fire Pokémon such as Flareon and Arcanine that I can rely on if I need a double fire move poke. I'm interested to see how valuable dual type movesets will be w PvP.

Based on the raw stats, I'd say FS is better, especially if you're using it as an attacker. However, if you wanted to put it in a gym, it may be a different story.

Obviously Charizard isn't a great defender to start with, but if you were set on using one as a defender, the pokemon that would normally be used against it would probably be Vaporeon, Rhydon, Golem, Jolteon. Of those, Vaporeon, Rhydon, and Golem resist FS, while Rhydon, Golem, and Jolteon resist AS.

It's a tiny difference, and likely won't matter in the long run, but in this overly simple analysis, FS vs AS would come down to whether you expect to be attacked by Vaporeons or Jolteons.