Worth powering this Charizard?

Personally, I would. Those Ivs are pretty good and Fs/O is a great moveset. So I think you should.

The higher CP one has to be at least level 36.5 or 37. Do you want to spend huge amounts of candy and dust to take it up maybe another 100CP, or do you want to spend the same amount of candy and dust to add 500-600 CP to the one with a better moveset and slightly higher IV's?

You are obviously a high enough level player to decide this for yourself, but given the upcoming gym rework, it seems pretty clear where you are going to get the bigger bang for the buck.

I would definitely focus on the FS/OH one, except I wouldn't do anything until the new gyms are introduced. You never know what changes are going to be made, and how moves, pokemon, etc are effected.

Charizard with such move set is the 2nd best fire attacker, so it's worth the powering up, unless you hate Charizard as a Pokemon. My 2522 FS/OH Charizard can take down a 3000+ Blissey by a few seconds slower than a Flareon does.