Fire question: which one to poder up

Moltres.

Would even do a lvl 35 eevee 89% before that flareon.

Max both. At least that's what I did.
But you could also wait for Heatran because we are far along into Gen 3.

If rare candy is not an issue, I'd go for Moltres. Flareon is too low level. But first you have to TM the charge move of either of them to make them optimal. Otherwise, there's no point in spending so much dust.

If you really need a fire attacker and can spare the RC, go for Moltres.