RE: ARM SOC vs x86
If you only care about the power consumption, I may have a suggestion for you. And that is not a difficult task for a linux expert.
You can use your modem for that purpose. May be you can find a better candidate for this purpose, but you can take a look at fritz!box modems and freetz firmware. It will cost no extra power consumption except the extra load on it. Furthermore, some models support USB storage expansion, so disk space will not be an issue either.
If you are not living in Germany, make sure you get international version of the modem.