It's hard to say. Mega Evolution makes most non-legendary Pokémon look unimpressive in comparison. Thus, many of the strong trainers of the anime's past retroactively look weaker than they were meant to be because the concept didn't exist back then. As we've seen in the anime, Megas are essentially in a tier above most regular Pokémon. Satoshi-Gekkouga isn't a Mega Evolution, but it's just as powerful as one, so it definitely comes across a lot more impressively that Ōba's Goukazaru did six years ago.
XY and
XY&Z have also, on two occasions, brought the Elite 4 and the champion "down to Earth", so to speak. TSME Act IV had Alan and Mega Lizardon outright defeat Pachira's Mega Hellgar, and XY&Z025 allowed Satoshi to go head-to-head with Carnet and actually get the upper-hand.
In comparison,
Diamond & Pearl – as well as the OS, AG, and BW – presented Shirona and the Elite 4 as invincible as far as Satoshi and co. were concerned. Satoshi was destroyed in battles with Kanna, Genji, and Ōba. He "did well" in comparison against Kikuka and Kikuna simply by virtue of being able to land a solid hit on their Pokémon once. Shinji and Shooti were both destroyed by Shirona and Adeku, respectively, and they were Satoshi's main rivals. It was pretty clear that Satoshi was nowhere near their level, and when a champion or an Elite 4 member came around, it was just to teach Satoshi and co. a lesson of some kind.
If I had to choose, I'd take Ōba's Goukazaru over Satoshi-Gekkouga. Even with XY&Z025 in mind, and even though Satoshi-Gekkouga will probably defeat Alan's Mega Lizardon (who beat Pachira's Mega Hellgar) at some later point in time, I still can't see Satoshi defeating a member of the Elite 4's ace Pokémon. He'd probably land some hits and take Ōba and Goukazaru off-guard (I imagine they'd be wondering what the hell Satoshi-Gekkouga even
is :p) at some point, but at the end of the day Ōba would win pretty handily.
Donno about that.
Ash's current team is a much better one, plus most of them are evolved at the time than Ash's sinnoh's team happened to be.
Plus, Ash worked on his own stamind, with his pokemon, and had few breaks when he was all about evolving his pokemons.
Edit: Plus he got one more slot on hand (And if you consider Goodra over there, this is not comparable anymore)
I think what
Meloetta meant by "more development" was that there was more focus towards their growth. Most of Satoshi's DP Pokémon had at least a couple episodes based on them learning to overcome their individual hurdles, learn new moves, or improve their relationships with other Pokémon or trainers. Goukazaru and Glion are the best in that regard, but Pikachu, Bouysel, and Dodaitose also had episodes centered around them. Only Fukamaru and Mukuhawk feel left out in that regard.
Satoshi's
XY Pokémon may be stronger and more evolved, especially so if Satoshi gets Numelgon back for the Kalos League, but they didn't have the kind of focus the Sinnoh team got. Satoshi's most developed, best-characterized Pokémon in
XY is the Numelgon he had for about 15 episodes.