The biggest advantages ARM has over Intel are all probably memory related. x86 will tend to have have more memory operations for doing a given thing, and with x86's stricter memory model it will be harder to design a system with lots of outstanding memory operations. ARM's simpler decoding is also an advantage from a power consumption standpoint, if nothing else. These aren't huge, though, and I'd give ARM a maybe 15% advantage or so? But remember that all things aren't equal, and Intel has a big lead in fabrication techniques over everyone else, and lots of huge teams of chip architects.