add my 2c.
OI is also has an impact on your decision.
When price FAZ and FAS are too big (such as b4 split, FAS 12 vs. FAZ 40), this has impacts on OI and vol (people always cheap and buy cheap).
At that case, FAS options vol normally much higher than that of FAZ. it means, for a trader, less price gap, a little easier to enter and exit. FAS option seems a better selection than FAZ's.