I actually think that the erratic strength of Ash-Greninja is a detriment to Ash as a trainer. He can almost take out the Champion's Gardevoir - and he WOULD have won if that battle had played out fully, let's not kid ourselves - but he can't handle one nobody's Mega Charizard X? Then consider that Gardevoir is part Fairy and Charizard part Fire. Hmm. Whilst he clearly got better at tapping into that power as time went by, he never reached that same level of power. He never brought out the full potential of Greninja, and he absolutely wasted his other Pokemon thereafter too - the anime devolved into "use Ash-Greninja to win" and little more.
Say what you will about DP Ash, but he's consistent - he improves as the series goes on, to the point that he can take down Legendaries by the end of it. He overcomes his major series rival - something XY Ash did not do, as Alain curbstomped him in the finals - and gets a noteworthy achievement as being the ONLY person to knock out Tobias' Darkrai...and a Latios on top of that, too.
I doubt XY Ash could do that, because his single most powerful Pokemon is one that he can't utilise properly. If he could, he would have stormed through every battle after Diantha, and he would have won that battle outright, too. He also refuses to use his old Pokemon, showing a distinct lack of flexibility, and he also lacks the creativity of DP Ash - he has no real strategies at all. His Pokemon are powerful, but he doesn't use them properly or to their full potential. Strength alone does not a good trainer make.
...and let's not pretend DP Ash wouldn't use his other Pokemon against XY Ash, hm? This "league teams only" nonsense is not reflective of DP Ash as a trainer, because he used a variety of Pokemon. That NEEDS to be taken into account if you're going to compare the two; it's not just about which Pokemon they caught in the region. After seeing his Sceptile barrel through Darkrai, I'd wager it'd do much the same to his Greninja.
I mean, would DP Ash REALLY use his Sinnoh team against XY Ash? After seeing him battle, DP Ash would throw a completely new team together - at the very least, Glalie would be in there. Sceptile too. Likely he'd rely on Staraptor or Gliscor to counter Talonflame, and of course Pikachu would be in there, and Volt Tackle is a hell of a lot more potent than Electro Ball. If he used Charizard, XY Ash would get demolished because as strong as Ash-Greninja is, very little gets by Ash's Charizard. Gible also has Draco Meteor, and I'd wager neither Goodra nor Noivern would like that.