Firstly, let's take a look at the matchups of the rest of the main storyline. Since you have Goodra and Aggron, I am guessing that you are either going to challenge Wulfric at least.
Wulfric: Both do okay. Staraptor has access to Close Combat which cripple Abomasnow and Cryoganal while Talonflame can obliterate Abomasnow and Avalugg with Flamethrower as Avalugg has low Special Defense.
Calem/Serena (victory road): Staraptor gets the edge here. It is great against Absol with Close Combat or U-Turn, can try to take out Meowstic with U-Turn and is immune to Shadow balls from Meowstic and Delphox. Talonflame is only good against Meowstic with U-Turn, which Staraptor can hit harder with.
Malva: Both are pretty useless in this battle, Talonflame can take resisted hits but will be destroyed by Torkoal's Stone edge and Pyroar's Wild Charge. Staraptor is immune to Chandelure's Shadow ball but also falls to Torkoal's Stone edge and Pyroar's Wild Charge. Talonflame is better here but it still stinks here. It just stinks a bit less than Staraptor here. (Just use Blastoise and you will be fine.)
Wikstrom: Talonflame is better. Reason being is that Wikstrom's Pokemon all have high physical defense to defend against Close combat, which Aegislash is immune to and is resisted by Klefki.
Talonflame, on the other hand, can hit hard with STAB Flamethrower which Scizor has a 4x weakness to. Staraptor is only good against Probopass and for resisting hits.
Drasna: Both of them preform terribly. They won't be happy taking a Thunderbolt from Dragalge of Talonflame taking a Surf from it too. Both Staraptor and Talonflame can't resist Drasna's other team members too, as they are special Attackers and Talonnflame and Staraptor have low Special Defense.
Siebold: Staraptor preforms better here as Talonflame is weak to water (but who would use Talonflame against four water types?) Staraptor can use U-Turn on Starmie and Close Combat on Barbaracle, but you have to watch out of it's Stone edge. It can also resist Gyarados's Earthquake when your electric type needs to heal. Sure, Talonflame could U-Turn the Starmie but won't take a STAB Surf too well. I am also aware that Talonflame can learn Solar beam in this gen (weird...), but it won't go well against Gyarados or Starmie, and probably can't take a Stone edge from Barbaracle.
Diantha: Let's go thru every Pokemon.
Hawlucha: Both do well as it is weak to Flying.
Tyrantrum: Staraptor is better as it has Close Combat. Watch out for Head Smash though.
Aurorus: Staraptor is better, again as it's Close Combat is super effective, this time being a 4x. Also Aurorus has a lower Defense than Special Defense, so Close Combat should be OK.
Gourgeist: Talonflame is better as it gets STAB type advantage.
Goodra: Staraptor is better as Talonflame can be taken out by Muddy Water.
Mega Gardevoir: Both can't take a Thunderbolt well but Talonflame has the edge because Fairy does less damage to Fire types. (But don't use it anyways use your Aggron's steel type moves.)
Overall, I believe Staraptor is better. Close combat is very helpful against many Pokemon and U-Turn is Helpful against Psychic or Dark type Pokemon. Even though Talonflame has higher Base stat total, Staraptor hits harder and is still decently fast. But what I think Staraptor is better at is that it is a physical attacker (yes Talonflame gets Flare Blitz but it only gets it as a level 1 move, it's best STAB move is either Flamethrower or Flame charge) and many Pokemon in game have weaker physical defense.