Hey i want to buy the game and i want to buy it on GOG since i despise DRM. However i have heard that it's better to buy the game on Steam. Is it true?

Not really it is the same game , same version and you can crossplay with steam users.. The only difference with steam is steam workshop and cloud saves, achievements (but this also has ingame achievements)
if you care about drm get it here, if not then you can get it from steam.

Steam offers cloud saves, but many players have complained about characters or worlds being broken if they use it.

There is no Steam Workshop for Terraria at the moment and the Steam achievements are the exact same as the ingame achievements.

Apart from DRM, the major difference from a typical end-user POV is that on Steam, tModLoader (the thing that loads mods) will be DLC. tModLoader for the GOG version will remain a stand-alone install. Not really a big deal.

My gaming LAN is not connected to the internet, so Steam just wouldn't work at all. Kinda like Galaxy doesn't work for me.