Hiya!
My 2¢...
Measurements: Use both. Put one in brackets. If that's "too much work", put it in Metric, but put a conversion table in the front/back. (e.g. "Plate Armor: Wt. 25kg [55lb]")
Spelling? Either or. Armor/Armour... Living in Canada most of my life I tend towards the Canadian spellings, obviously, which includes ample use of the letter "u" as well as switching "el" and "le", or "er" and "re" around at the end of some words.
I find I use both. I like my "general ht/wt of people" to be in feet/pounds, but I like my longer distances in kilometers, as well as speeds... shorter distances up to about 300' or so in, well, feet. Oh, and I like liters in stead of gallons. Small distances I seem to use mm/cm and inches/fractions more or less interchangeably (e.g., "The book is about 11"x8", and maybe 2cm thick"...yeah...nutty...teach me to live the first 1/5th of my life in the US, in the US school system...then the rest in Canada! I'm so conflicted with my measurements!... LOL! ).
^_^
Paul L. Ming