Well, if the performance and battery life were equal, it would probably come down to price; if price is also equal, then I'd likely go for x86, as there is more binary-distributed software I might wish to run for x86 than for ARM, and I don't intend to do any assembly programming on either.

OTOH, I might go ARM, just to support proper design, and because I like it; if you've even looked at ARM and x86 instruction sets, I can't imagine liking x86 better.

That they don't (and won't ever) have equivalent performance and battery life to the best RISC CPUs, and so aren't really worth discussing unless you need legacy x86 binary compatibility.