A quad-core Xeon is overkill for a static content webserver. An ARM is overkill for an embedded controller. If you specify inappropriate hardware then you'll end up with an inappropriate power/performance ratio.
As for low-power x86 - HP's Moonshot is in the ballpark of ARM blade devices, and Baytrail pushes Intel even closer. ARM probably still wins, but the figures are nothing like 15x. And once you take fixed costs like disk and RAM into account, the difference ends up being even smaller.
ARM have done a great job of improving the performance of their cores. Intel have done a great job of cutting the x86 power budget. Given that nobody's really shipping ARM servers yet, it's still not clear who's going to come up with the better product. The problem that ARM face is that they not only have to be better, they have to be sufficiently better that it's worth the cost of porting in-house applications to a new architecture.
As for low-power x86 - HP's Moonshot is in the ballpark of ARM blade devices, and Baytrail pushes Intel even closer. ARM probably still wins, but the figures are nothing like 15x. And once you take fixed costs like disk and RAM into account, the difference ends up being even smaller.
ARM have done a great job of improving the performance of their cores. Intel have done a great job of cutting the x86 power budget. Given that nobody's really shipping ARM servers yet, it's still not clear who's going to come up with the better product. The problem that ARM face is that they not only have to be better, they have to be sufficiently better that it's worth the cost of porting in-house applications to a new architecture.