I know GOG is DRM free, but is that the only difference from the GOG version and the Steam version, and does that even make a difference?
As with all things, the more complexity there is to a system, the more quickly it will fall apart. Lets say Steam is purchased by AT&T... the USA's most hated company. Do you think your game will be safe in their care? Probably not. And this happens all the time in the business world. Businesses change hands, new executives are hired, owners change the rules and the consumer has almost no power over it. Ive been around a long time, never let someone else guard your assets. They will try to steal them (or find ways to charge you more for them) eventually.
Always take the form that gives you more freedom. Dont worry about mods. Popular modable games always end up Nexus which is 10x better than steam in practically every way especially licensing. They will likely be available on the forums as well.