Charizard or Arcanine
I want to power up a fire type mon (not flareon). My better options are a level 24 Charizard with 93% ivs and WA/FB and a Arcanine level 20 89% with FF/FB. Both of them are "perfect", I know Arcanine is stronger and have a better max CP than Charizard but the level difference and Charizard being Charizard would be enough to make Charizard a better choice? Candies are not a problem, I already have enough to take charizard to level 30 and growlithe is not so rare in here.
Answers
Your best choice seems clear to me: Charizard.
Better IVs, less stardust required, coolest mon, same use - maybe a bit more versatile because it'll be super-effective also against fight types with WA.
I personally have a Charizard 93% like yours at lvl 32 and enjoy to play it vs Exeggutor. Also have 4 Arcanines with >90% IVs and optimal/decent moves and never bother to power them up. Fire types make gyms quick and easy to take down.
I'd invest my stardust on good defenders and pokemon I like. Arcanine may be a good place-holder in gyms because of high CP, but it's an awful defender...
I wouldn't power up both. Not worth the stardust, in my opinion. If Growlithe is common in your area you may wait to get a higher level wild growlithe with good IV. In the meantime your arcanine lvl 20 is still usable to take down gyms - may just have to dodge a couple of times ;)