I'm a fan of Steam and PC games but I don't know about these.
They just seem like nothing more than buying a Linux based PC. I priced out the same components and by obviously choosing the cheapest of each I came out to around $650 without an OS. It also seems like you are in for a big disappointment when that Steam machine you bought won't play the game you want because you didn't get the one with X inside. Sure you can just buy that upgraded part and put it in yourself but I can do that with my PC already. There is a reason gaming machines cost a lot and these seem like low powered machines made to run sub AAA titles. A console gamer knows when he puts a game in it will run as expected without tweaking the graphics settings. In these it sounds like, depending on which one you get, you will have to tweak making them more confusing to people who may think they are buying a PC console.
Something else I wonder about is the Linux OS. Is it able to run any game Windows can or does the game have to be ported to Linux first? Being a Windows guy I really don't know how that works but I would be worried that manufacturers wouldn't try to optimize the Linux drivers for the best performance since they are not their #1 customers. I know of Valves hatred toward Windows, not to mention the added cost of bundling it into a system, but it seems like you are putting a lot on the future of gaming with Linux.