I've never used D2D, but looking at their web-site, I don't see how they're essentially different from Steam: it's still all download-only.
In fact, with Steam it's easy to "back-up" your files onto CD or external hard-drive (right click on any title in your library), and I've moved computers several times, and redownloaded my games with no problem anyway. Steam also has the advantage of in-built chat and it pops up "XX is playing Monkey Island 2 Special Editon" on all your friend's computers, which looks quite cool.
However, Steam can be annoying for some people. It is a myth that you need to be connected to the internet and logged in every time you play the game, but though that isn't true, Steam does encourage you to operate like this, and it also encourages you to use it to launch your games. In other words, Steam likes to take over.
D2D is also 4p cheaper. Think of all the penny sweets you could buy for that! (The answer is four).
...but if your main concern is having a secure version of the game, I would go for Steam, and try and back that up on an external medium.