I tend to agree with @txa1265 and @Andrew23 about the updates. GOG made efforts but updates are regularly earlier on Steam, and I have the impression from others' posts that they're smaller. That being said, GOG offers a lot of flexibility in the temporary directories, which helps sometimes (I was running out of SSD space once and it saved me).

Regardless, GOG is my first choice because

• it doesn't force to update,
• the client is lighter (even optional) and doesn't update itself all the time,
• it's a guarantee of DRM-free content. No nasty Denuvo there

(btw, there are achievements on GOG too, though I never compared them on both platforms).

So I'm not entirely sure which games they have in mind for their shortlist, but my personal hope is that they keep good, new games as well as "refurbished" ones, even if only by indie devs (I don't care about AAA games).