You should go with your first choice, since it's pretty clear that you don't have anything below that can interfere. But when you look at your mounting location, also look what's around that also might block the signal. With a survey, you can determine good locations for mounting especially if there might be things that attenuate the signal.
What I have done in the past also is had the low voltage guys run a conduit down to a certain allowed height above finish floor and have a single or double gang box at the end of it. Then you can use the universal AP mount and attach the AP to the gang box.
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-Scott
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