I personally like GOG more than Steam for three big reasons:
1: You don't have to run a client in the background.
2: Their customer service is awesome, and they genuinely do seem to care about you (they actually refunded me money when I couldn't get a game to work on my rig).
3: You can share games with friends. Since you literally download that actual game file to your computer, you can sign in on your account on a different computer and download any games you've purchased (or just give them your username and password).
1: You don't have to run a client in the background.
2: Their customer service is awesome, and they genuinely do seem to care about you (they actually refunded me money when I couldn't get a game to work on my rig).
3: You can share games with friends. Since you literally download that actual game file to your computer, you can sign in on your account on a different computer and download any games you've purchased (or just give them your username and password).