I would personally love to use GOG for their buy-to-own model, but I'm incredibly tied into the Steam ecosystem. I just can't live without Remote Play Together for playing with distant friends, the Workshop is incredibly convenient for modding, and free no-setup cloud sync of all my saves is a no-brainer. Gabe Newell was right when he talked about piracy being a service issue. If you provide the best service, people will keep coming back.

In that same vein, I'll never buy another Ubisoft title as long as I live. Their crappy launcher makes it impossible to play their games on Linux.

I tend to not buy a game on Steam if I already own it on GOG and vice versa. Why pay for the same thing twice?

Steam. Valve pumps money into Linux gaming, and it shows by games running really well with almost no effort on my part. I've also had issues running controllers on GOG through Heroic (maybe fixed?), whereas it's flawless and incredibly configurable on Steam.

If GOG starts to give Linux actual attention, I'll prefer them for their DRM-free stance. But my gaming experience is just so much better on Steam that I don't bother with GOG most of the time.

I tend to prefer the Steam version if I have both, mainly as you say for acbievments and keeping my playtime all in one place. But also the community forums and guides are nice sometimes, and easily accessible with the steam ui.

GoG I'll pick (and mainly purchase towards) older games and nostalgic favorites where lack of drm feels more important to me. Those often don't have achievements and such anyway.

Funny enough, I get pretty annoyed about that playtime tracking thing. I wish there was like a last.fm scrobbler but for games. Before my SteamDeck, I rebought several games on Switch because I liked the portability. So now my playtime on those is split across multiple devices. Ah well, truly first world problems.

For me it's GOG first. Using lgogdownloader and wine directly (in a custom apparmor profile). No DRM, no forced updates, no annoying client that takes forever to start. Games are also dramatically much easier to isolate and sandbox this way.

If the game is not there, then yes, Steam (as a separate unix user).