mindblast: Well, here's my take. Buy whatever games you want on whatever platform you think it's more cheap, maybe take benefit of sales, or, go old-school, buy a game, play it, finish it, play it few more times, then pick your next game.
Today, as we're one click away from cheap games, it's hard to resist all bundles and price drops, but, a lot of games, at full price, especially on Steam, they are very overpriced. I mean, on many few years older games that are still available in physical stores, prices on Steam are even 10 times higher than what i would pay for an physical CD.
Now, other thing as a comparison is how the game is patched in order for you to play it. And this apply mostly to older games. With Steam, that patch it's pretty much nonexistent. You kinda get the game as it is on CD. You might need to use DOSBox or to modify the game in order to adjust it for your needs. GoG, on the other hand, patches the game for you. On older games, you have DOSBox included in the package, linked to the game executable, so you don't need to hassle with it. From my experience, any game i tried on GoG worked flawless. On Steam, on the other hand, i had encountered a bunch of games that were not running, i needed to either modify the game files or just to abandon trying playing those games. Now, lucky for me, i got a lot of games due to promotions and bundles, but, if i would have payed the full price, i would have been very mad.
Then, there are the extras. On many games, GoG offers all kind of cool stuff beside manuals. Wallpapers, artwork, soundtrack, books, maps, etc. Steam does not care about those. They even sell the soundtracks separately for many games, while on GoG you often find hem included in the original package, that is, most of the times, at the same price as it is on Steam.
Now, not to bash Steam too much, it's a popular platform. You find a lot of deals there or through bundles, that seem to prefer that platform. I personally don't have the problem with their DRM. Also, a lot of new small, indie titles, are promoted through Greenlight and released on Steam. There are some gems that were launched due to Steam and i'm grateful to them for that.
To cut story short, i would say don't bother too much. Use both platforms. If i would want to buy a game and i would have to pick between GoG and Steam, if they are roughly on the same price, i would pick GoG right now. But other than that, if i find good deals on Steam, i get them there, especially on newer games.
Regarding the extras you speak of (soundtracks, wallpapers etc.), these things are more than welcome as freebies but not as DLC in a so-called "Deluxe" (or whatever) version of a game. They are just fluff. Most people don't download them anyway and would rather not be treated like idiots. To charge extra for them is more than just a bit cheeky.
Steam is still an abomination in my eyes because when I started gaming there was nothing like it. You shouldn't have to be beholden to a "client" to play a game once you've paid good money for it. I buy games from Steam which I can't buy on GOG, and only if I think they'll never come to GOG or will take aeons to appear.