Thought of an even better problem with interpreting it exclusively.
If it is to be exclusive, and both armors would cause encumbrance, which applies? SR4A does not say to choose the higher of the two penalties. It does not say to choose the higher of the two armors, either. It is simply that you have chosen, without rules-guidance, to select the higher of the two armors.
But, since it says "if either of the armor ratings" (to paraphrase), you are required to test both armor ratings against Bod x 2 (normally), which then precludes selecting the higher armor ratings. So you've tested both, and both indicate that you should have an encumbrance penalty, determined by the difference of the armor and Body x 2 (normally), divided by two.
Do you test ballistic and stop testing if Ballistic > Body x 2? What if you started at Impact? If I test both to preclude someone avoiding a higher penalty, then I have no guidance on which penalty to apply, except the general approach of applying both. This is the only solution which does not require putting words in the mouths of the authors about which armor to test.
I agree, it would be stupid to use the lower of the two armors if it was intended to be an XOR situation. However, the simpler solution is to address both possibilities and combine the modifiers. This is consistent with wound penalties (you don't take the greater of Stun wound modifier or Physical wound modifier, you take both).