The services are fairly similar, really. Yes, the Steam client is a gateway DRM. Yes, they sell many games that are DRM-free once you download the installer via that DRM-layer of the client. If you're good with that level of DRM, then the differences start to disappear. It comes down to the details and what you prefer.
Both have refund policies, but handle them differently. Steam lets you return within two hours of (tracked) gameplay time, whereas gOg is calendar-based (that's a big generalization, as there are many other details)
Both offer big sales.
gOg is selective in what shows up in the catalog. Steam appears to take on any- and every-thing that shows up on its doorstep. gOg's selection size thus suffers but it could be argued that the overall quality level is higher, and it's easier to find good games here with less dross to sift through.
Steam's client is mature. gOg's is beta and has (apparently - I've never tried it) a long way to go but it seems to function on a basic level. gOg's is also optional, and thus is not a mandatory additional layer.
gOg has an active support staff that tries to solve game problems. Steam's support might be limited to account / transaction matters - I'm guessing on that since I don't use Steam, but people generally praise gOg for having deeper support.
gOg tests each release on various configurations, to try to find problems before a game hits the catalog. And, of course, they work with the pubs / devs of old titles to get them to work on today's hardware and operating systems. It's not perfect, but the track record is impressive.
Game updates are released first on Steam. Their backbone allows dev/pubs to directly provide updates, whereas updates at gOg get some testing and such before being released. Given that it's easier for dev/pubs on the Steam side, those customers see the updates before we do. If you don't mind waiting, then it's not a big deal.
But DRM-free is also holding back AAA releases. That, and Steam offers the backbone for updates, DLC, etc, that automate things for the userl which in turn makes things easier for the dev/pub. gOg is more hands-on for the user and the dev/pub. Both have their proponents and detractors. Being an old-school gamer, I prefer gOg's approach (not that it's perfect, but still).
In the end, they both sell games. Prices are similar. Once you have the game installer, the DRM-free games should be all-but-identical.
As you say, the sale is a good time to try out the service. Hell, that huge Interplay dump should be reason enough to give it a try. And don't forget that they offer a good dozen+ free games so you can try the service risk-free:
http://www.gog.com/games##sort=bestselling&price=free&page=1