Just my two cents regarding Paul losing to a BF brain, it seems you're devaluing how strong they are. If i remember correctly, they are between Gym Leaders and E4 members in strength and Brandon in particular is on par with E4 in my opinion. I mean it took Ash 3 (4?) times to beat him. So I'm afraid I dont see the point in saying Paul lost to Brandon when comparing XY Ash and Paul. A large majority of trainers lose E4 and equivalent trainers all the time, so it'd make sense for Paul to lose to Brandon. So using that argument is pointless considering XY Ash hasn't beaten anyone of that caliber either.
Now I concede that Alain is a strong trainer and a close loss to him is nothing to be ashamed about, but he is not an E4 caliber trainer. His other Pokemon are fairly disappointing when looking at how strong his ZardX is, I would've liked to see his other Pokemon be more impressive. So in the end the two are pretty much even when it comes to most impressive wins.
So it comes down to team makeup, battling abilities of said Pokemon and the strategic ability of the trainers. Now I voted for XY Ash but the battle could absolutely go either way. One thing I like about Ash's team is that his team, in comparison to DP Ash, sets up nicer against Paul's team. Toxic Spikes no longer cripples the team, he's got more special attackers that will help with keeping Paul's physical attackers at bay a bit more (not to mention his flying types making it hard for the majority of Paul's team to hit them) and honestly, I like Goodra in a battle against Electivire IF it happens which takes out Paul's biggest threat to Ash.
Like I said, the battle is too close to definitively call though because Paul is one of the best strategists we've met in the anime and XY Ash is arguably the best version of Ash when it comes to pure battling ability, though his fly by the seat of his pants style can be a bit hit or miss in the bigger battles.