With Steam having effectively cornered the market as the online distribution system for gaming, perhaps this is a moot discussion, but I think it is still a topic worthy of debate.
I suppose the most obvious question to most people is "what is Impulse?" Impulse (from Stardock) is another gaming distribution system that is very comparable to Steam (from Valve). The two systems have many of the same games for sale, and many that are exclusive to their own respective platform. I will say that Steam does have the larger selection of the two. Aside from the actual games offered, there are a few important differences between them.
The biggest difference is that Steam requires that the client is running in the background in order to play a game, while Impulse does not. This may not seem like a huge deal at first, but the implications of this really are a huge deal. First, any game bought via Steam becomes inextricably tied to the client. They cannot be separated.
Another big difference is the file structure. Steam places all its games within its own program directory and many of the files are buried within its own proprietary container. This makes accessing individual game files an enormous headache and makes any kind of modding a near impossibility. This leaves one with the impression that games bought through Steam belong more to Valve than to the user. With Impulse all games are installed in the "Program Files" directory, just as if you installed the game directly from a store-bought CD or DVD, and the files themselves remain unaltered. The games you buy are your own to do with as you please.
To Valve’s credit, Steam has been released for the Mac, making it the only game distributor to do so (that I am aware). Stardock has always said that it is and will always be Windows only. There were several rumors recently that said that Valve was planning a Linux client but those were squashed.
So what does all this mean for an end-user? Well in the end when all one wishes to do is play games, not much I suppose. Having to have the Steam client run in the background might cause some difficulty for older machines. I can imagine some graphics options being needed to be turned down or off in order to accommodate the extra running process. But in the end both systems do work very well, so it is a matter of preference.
I must note however that there are some other considerations to be made from a Linux user’s perspective. Neither have a Linux client, but Steam does run under Wine very well. I have never been able to get Impulse to run under Wine. However with all of the game files at your disposal, it is not necessary to run Impulse as long as you have a Windows partition or in a virtual machine where the games can be installed and migrated over.
All in all as a personal preference I love the Impulse system. If a game I wish to purchase is available from both, I will get it from Impulse every time. That being said, I have a few games that I bought via Steam as they were unavailable from Impulse, but it does seem to me that Impulse is getting bigger all the time.
I just wish one of them would develop a native Linux client and start to port over some games.