I'm gonna argue for Ninetails, following are my reasons:
- It gets Drought, which means all it's fire moves are stronger
- Drought also means that it can wield Solar Beam with great success
- It gets Extrasensory which kinda makes up for the Psychic type you are missing
- It does not add to a Dark weakness in you team. A Psychic type is not a necessary thing to have in your team, and if you really want one, Ninetales has Extrasensory as stated above.
- Delphox has a very limited things going for it. It's frail and lacks the firepower that Ninetales has.
- Since your team already has two frail and fast hitters (Mega Gengar and Cinccino) I don't really see a reason why you'd want another.
Here's a set I would recommend:
Ninetales @Heat Rock
EVs: 252 Sp.Atk / 252 Spe / 4 Def
Fire Blast / Flamethrower
Will-O-Wisp (for crippling things like Tyranitar and Mawile)
I have the audacity to actually say neither. This is because both Pokemon increase your weakness to Ground (3 Pokemon weak to ground and not one resisting it, bar the pre mega-evo Gengar) which is a pretty serious weakness to have.
And since you want a fire type on your team, here's the best one you'll get out there which won't increase your weaknesses.
EVs: 252 HP / 252 Sp.Def / 4 Def
A specially defensive Rotom-Heat, as your team overall lacks bulk aside from Ferrothorn, which is physically defensive. Overheat hurts and dents a lot of things that don't resist it. Here's a sample of what it's capable of:
>0 SpA Rotom-H Overheat vs. 0 HP / 4 SpD Mega Lucario: 390-458 (138.7 - 162.9%) -- guaranteed OHKO
>0 SpA Rotom-H Overheat vs. 252 HP / 240 SpD Filter Mega Aggron: 202-238 (58.7 - 69.1%) -- guaranteed 2HKO
Volt Switch to help you gain momentum and find good chances to bring in your fast Pokemon. Will-O-Wisp to, again, cripple threats. Pain Split for a reliable means of healing.
The big reason of choosing this is that it walls Talonflame to death, which your team is quite weak to aside from Tyrantrum. There's absolutely nothing that Talonflame can do against it, and is completely countered.
Hope I helped!