PookaMustard: Nice question here.
GOG wins hands down though. So let's begin. What does Humble Bundle has? That's right, the bundle part. You get games bundled in a... bundle you see at a cheaper price. The other advantage being that occasionally, they also sell Android games, and the PC and Android Bundles are quite the best pick-ups for being DRM-free, for all PC platforms as well as available for Android. And that's it.
Humble Bundle, 80% of the time, will put up a bundle that is Steam keys only. Or the DRM-free game being the one on the top tier, in which case buying it on GOG is a better investment. Humble Bundle also for some reason, can't let me log in with either Internet Explorer or Microsoft Edge, forcing me to get a secondary browser just to download or buy the games there. Other times it'll be a bunch of DRM-free but more Steam keys. It is inconsistent as you see.
Not long ago, there was a bundle where the Steam keys beat the DRM-free games in total. Right now, there is the Square Enix bundle, all Steam'd to hell. Not even longer ago though, the NEO GEO bundle had more DRM-free games than Steam games (exclusive or not).
Add to all of this the slow support, them blocking bitcoin payments for the same bundle if another payment is made in the next 24 hours after the initial one, the clunky website structure, and updates being not as visible, and its not pleasing. Though kudos for the bitcoin payment feature.
On GOG's side, ALL GAMES ARE DRM-FREE. None of the madness Humble Bundle suffers from. You buy it from GOG, its DRM-free. Don't think about it. Not only that, they come with their extras. Why should I stop here? Their support is outstanding if what I hear here is any indicator. GOG optimizes their games to run on modern operating systems. Their website is less clunky than Humble Bundle and can let me log in from any browser. If a game receives an update, it puts a notification on my Accounts tab and lets me know which game got an update. And then I've got GOG's own community features, friends, chat, forums, whatever.
I still say Humble is better. Don't want the Steam keys? Don't use them! Or just give them away. It's still much cheaper for the games you ARE getting.
And honestly, the more I think about it, the more I realize that the lack of decent support doesn't really matter that much and I can see why they don't care. Why am I going to go whine about one game that I can't get to work when I already got $50+ worth of games otherwise? If I can't get that ONE to work, they're not about to refund me for the whole bundle. That's ridiculous. Plus, if you're not paranoid about DRM, you use the Steam key as sort of your "backup" copy.
And seriously, who cares about bitcoins? I didn't know people actually used those things.