In any way? DRM is obvious a plus but I wonder if there's a downside to it.

Unfortunately, some users have reported that the currently available version on GOG is afflicted with a memory leak issue.

However, LoRezJez had mentioned a work-around (involving the GOG Galaxy client) that may be of assistance (in the event that you choose to purchase the game on GOG).

The downside I'm aware of is that some modlists such as Lorerim only support the Steam version.

The only downside that I've run into is that there is no Creation Kit for the Gog version, so you can't create Mods with the Gog Version. The plus side is that the mod creators pretty much support the GOG version (I have 816 active mods).

Even that problem can be resolved provided you also own the Steam version or know somebody who does. I have created mods using the GOG version.

There is a guide here on how to mod the CK to work alongside GOG:

https://www.gog.com/forum/elder_scrolls_series/skyrim_creation_kit/page1