Initially the two were pretty different. Steam did its new-releases and Greenlight thing with DRM, and GoG released retro titles with built-in DOSbox emulation DRM-free. If not for the fact that DRM-free releases are a deal-breaker for most publishers which keeps a lot of titles off of GoG, I'd have no reason to make many purchases through Steam in the future. Unfortunately Steam is still the AAA and indie lead while GoG handles the retro market. Currently it's a PC ecosystem more or less, and I don't have any problems with that. Now in a perfect world publishers would stop holding titles hostage with DRM, something which would be good for all gamers, and that would allow GoG to go from a separate corner of the market to being able to wage 1:1 competition with Steam.
Overall, despite having fewer titles than Steam, I much prefer GoG solely from a business ethics perspective. CDPR released Witcher 3 through GoG, DRM-free and with free DLC; did CDP/R go bankrupt the next day? Be nice to your consumers: What a novel idea.