I always choose buying on GOG if I have to pick between both because of no DRM,of course,but I see that there are some games on GOG that got a different deal of content on Steam only,like Psychonauts(Steam has an exclusive update that fixes the game even more.I think the same update is on Humble Bundle too),The Last Express(has a Gold Edition avaliable on Steam with new features),Baldur's Gate(GOG doesn't have the Enhanced Editions.).
The inverse applies as well,since you can buy the whole Broken Sword series(including the original unaltered versions of the first two games as well)here on GOG while Steam doesn't have BS4(sure,BS3 and BS4 are the weakest of the series,but I like to own the whole series anyway...) and only the Director's Cut versions of the first two games,and there's also Fahrenheit,which GOG sells the original uncut version,while Steam sold the Indigo Prophecy version(and it's not even avaliable os Steam anymore,anyway.)

Myst - GOG has Myst - Masterpiece Edition and realMyst but Steam has realMyst - Masterpiece Edition!
There are also HD versions of Serious Sam 1st and 2nd encounter and MDK2
I think their Duke Nukem is slightly more funky and source ported out of the box too...

Duke Nukem 3D on Steam also has the three expansions, while we have none here...

But .... you are not even one day old, are you a Mayfly ? (edit : a very active one, your knowledge of gog titles is impressive given your age) You don't want to open that box.

This is an interesting point though, I would have thought it already had been discussed but didn't find any trace.

Welcome and enjoy the stay :)

I used a poor choices of words(English is not my first language.),but what I meant is:I researched the benefits of GOG,and I am already buying some of the games,but I noticed that Steam still has some different content like I mentioned before.
It is interesting if we make a complete list,not to say which version is better(Steam or GOG)but for informative purposes

And now Serious Sam Classics: Revolution (free for who owns the classic 1st and 2nd encounter)

I know 7th guest and 11th hour on steam all have the extras and are DRM free.

Wait? WUT? this is an even more HD version than the HD versions but only free to owners of the classic versions? This seems odd...

"Surgeon Simulator 2013": Steam version has an extra operation involving TF2 characters.

They do? Those assholes.

Yes. What I did was copy the game folders to my 2nd HDD, closed steam, ran the scummvu thing for 7th guest, it ran without steam, and just ran the game exe for 11th hour and it also ran without steam.

GoG or Steam ? :)

Don't answer ! they'll break the 2 !