I just buy where a game I want is on the lowest sale at the moment, but still check if a game is properly updated on gog first, since some games haven't been updated in a long time on gog.

Tho I try and mostly focus on old games not on steam anymore or indie titles are are updated on parity with the windows release, otherwise I prefer steam, also cause I own a legion go with bazzite installed so it's easier to play games like that.

The only black-and-white rule is: if you're buying an older game you should always buy it on GOG. No exceptions. There's too many retro games on Steam that won't even launch on modern machines, and beyond that GOG is typically very good at including fan made patches and fixes into their versions of old games, ensuring older games actually work and are just plug-and-play.

Every gimmicky feature on steam is fluff to lock you into an ecosystem. Buying steam games for achievements for example is really silly.

Truth is if you care about a sustainable games industry, then drm free and maximising developer cut is the right choice.

Choose the option that gives the developer the biggest cut, like itch or directly, and just use a launcher aggregator to manage it from one place like heroic launcher or playnite. I personally use gog for most but will use an alternative if it supports the dev better.

I buy most co-op ("always online" like Darktide or Helldivers 2) games on Steam (, ...) since they're often not available on GOG

For single-player, I prefer to wait for it to be on GOG. Sometimes it's on release, other times it takes a few months (Expedition 33) or years, but I have plenty enough backlog

I've been a Steam only buyer for a long time. There are so many cool features and extra stuff, most games work out of the box, and they've been putting in a lot of effort on the linux scene with proton and the deck. But even despite all that, I'm starting to move to GOG. The sad truth is that you don't own any of your games on Steam. I've been having more and more games be removed from my library, and games that either just don't work or are "updated" into something worse. Not Steam's fault really, but GOG is much more consumer friendly and I actually get files I can use and keep forever, no required updates or DRM. I really like Steam, and am having a hard time leaving it, but GOG is just the better choice from a long term and consumer ethics perspective.

How does one buy games with trading cards? I have tons of them

I use a decision tree to determine which platform to buy from, by going down this list and selecting the first platform that fits my requirements.

1. Steam: If the full game is on there, buy. If the game needs an extensive patch to provide all the content, buy from another platform.
2. itch: If the full game is on there, buy.
3. GOG: If the full game is on there, buy. If the game needs an extensive patch to provide all the content, buy from another platform.
4. JAST USA: If the full game is on there, buy.
5. DLSite: If the full game is on there, buy.
6. DMM: This platform is annoying to buy from since they dislike foreigners. If the full game is on there, buy.