First off, sorry if this has been asked elsewhere.

Now, on to the main subject: I was wondering if there is any reason to pick the steam version over the GOG one or vice-versa, or whether it's purely a matter of preference? Have they both been updated/patched to the latest version? Are there any glitches/bugs/errors which are exclusive to one version or the other?

Obviously, the GOG version is my first choice; I just wanted to know whether there was any reason not to buy it.

As far as I know, the GOG version is up to date. But why would you ever want to by a game from Steam if it is available DRM-free?
To answer your question: No, there isn't any reason to buy the Steam version. Buy it here. Also, I think that you get some bonuses that you won't get on Steam like community artworks.

The Steam version does include achievements that the GOG version lacks. The majority would be picked up just making progress through the game, but a few require exercising optional and sometimes obscure portions of the dialogue tree or visiting or revisiting locations that are somewhat off the beaten track.