A word of warning. Get in the habit of thoroughly commenting your assembly early on, or you likely never will. If you start by commenting too much, you can gradually bring it down. If you start commenting sparsely (or like me, not at all), it's considerably more difficult to increase it.

That said, you picked a good first ISA to learn seriously. ARM has some very cool features, even if it does have its serious faults (poor floating point/vector support, even where it's available). Learning x86 will be trivial from there; the only difficult thing is knowing what the side-effects of functions are, as x86 is horribly inconsistent in that regard.

Good luck!