No reason not to go with a 9s chain if the price is comparable. As you point out you'll end up with some spare chain which might be useful one day.
As for rotating the rings front to back, that's a waste of time and effort. The wear rate of both timing rings is about equal since the chain load id the same on both. However chain load varies throughout the pedal cycle, and is highest when the cranks are horizontal. So what you might do, is advance the rings by one bolt position. (ideal is 1/4 turn but that's not an option). If you periodically advance the rings one position, you'll even out the wear through their lifespan.
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