I would personally wait until the end of the event, because you could possibly get a better Charmander (for example, same IV, higher level). And you will have a lot more Candy at that point so evolving that other Charmander will not be that costly anyway, so you can do it and pray for the moveset you want
Charizard Power Up Decision
Say you have decided you want to power up a Charizard to max. Your level 31 and have a level 20 Charizard with 93% IV and WA/FT. Say it's the day after this event and you have 170 candies. You have a 96% IV Charmander at level 20. Would you evolve that Charmander in hopes of getting WA/FB or just dump the candies into your existing Charizard.
Answers
I always thought FB was his best charge move with WA. I don't have much experience with FT because the Flareon and Arcanine I powered up have FB as I believed it to be the better move. What makes FT better than FB? I get you have a better chance to dodge an opponents charge attack if they use it right after you use yours but is there anything outside that?
Difference between Fire Blast and Flamethrower in DPS isn't much. With FB you get extra 4-5% DPS, and with FT you get a little bit of extra dodging ability (but honestly not much - it's not fast enough to dodge the next attack most of the time so you're eating it either way)
Extra 3% IV (aka 1 stat) isn't going to make much of a difference. Certainly not noticeable in battle. If it were a defender every CP counts, but it's not.
Honestly it's really personal preference at this point. Use an arcanine with both charge moves and see which one fits your play style better.
But at the end of the day, the event's not over yet. So what's the rush? I know you're excited but you never know what you'll find before event is finished :)