Both Showdown and cartridge ladder have their own benefits.I always hear VGC players shittalk the Showdown ladders - I really wonder why. Is it true that the quality of games are better on cartridge? What do you guys think
Showdown is great for testing teams and finding out what you're comfortable with before obtaining it in game. It's much easier to make changes to your team on showdown, and you can play more games in a given amount of time allowing for you to get more experience. The UI displays a lot more information making it easier to understand what's going on, and there are replays so you can watch back your matches and learn from them - something which on cartridge requires a capture card and significantly more time.
Once you have your team mostly finished and want to test it to see if you need any final touch ups, cartridge is likely better. There's a wider variety of players in terms of region on cartridge. In particular, you will find many more Japanese players on cartridge compared to Showdown. Since the Japanese metagame has noticeable differences from the western/"Showdown" metagame, it's essential that you get cartridge experience against a wide variety of players. Not to mention cartridge is what you'll be playing on for official tournaments, so getting used to it and learning the nuances of the UI and what you need to remember yourself is essential to maximize your performance.
As far as quality of the games goes, it really depends. Everyone on showdown is here to play competitively. Because it's easier to switch teams, you'll see more gimmicky teams, but that's typically at lower ladder and they at least have an idea of what they're trying to do. Since the cartridge is also in the hands of people who know little-to-nothing about competitive Pokemon, you will often in the early tiers get opponents who don't even have their Pokemon EVed or are using their first playthrough teams. Once again there's pros and cons to both.
I think most top players (at least western ones, showdown isn't as popular in Japan) utilize both Showdown and cartridge. Typically, when people dislike Showdown it is a moral disagreement or they are not used to the platform. Ex. something I see commonly is people saying that there is more hax on Showdown, failing to account for the higher volume of games which means there will be more hax naturally. I think this is a perfect case of someone who isn't used to the platform. As for morals, some people don't like supporting unofficial parties. To each their own.