If I were you I would go for #1. Assuming you have the TMs to deal with movesets, that 15 Att is a big selling point. Also 13 sta isn't bad either. But if you want the CP, of course go for the 96%, I don't even think there is that much of a difference between 13 and 15 Att. Also #1 saves dust and candies. So I don't know what your stardust or rare candies look like but that may be a big deal.
which Moltres to power up?
1) Moltres, lvl 20, IV 87%, atk 15, def 11, sta 13
2) Moltres, lvl 15, IV 96%, atk 13, def 15, sta 15
3) Moltres, lvl 15, IV 84%, atk 14, def 14, sta 10
normally I would go on 2) without thinking, but all those big assuptions on ATK made me esitate.
Answers
Don't power up Moltres before we know how good community day Charizard ends up being. With a charge move as good as Frenzy Plant, it would have higher DPS than Moltres, which makes investing into Moltres or any other fire mon a bit risky at the moment.
As for the actual question, 1 will save a lot of resources if you only plan to power up to whatever is the relevant breakpoint. I would go with 2 only if you wanted to max it eventually.
For example, I live in SoCal, and the weather is often Sunny here.
In which case Moltres with FS/OH Out-DPSes many options and is even viable for targets like FB Mewtwo.
Also, that is subject to change, especially when the Regis enter the scene. Moltres will be highest DPS (though perhaps edged out by BB Charizard) against Registeel and at least somewhat viable as a glassy counter against Regice.