If there's another single (and yes, it's still an if now), I don't think she can go wrong either way. They are just as divided and have went back and forth between the two for the last month.
Look, one of the pros of releasing "Walking On Air" (which could be a remix) is that the big stations would finally get what they want, and people do seem to really like it. At worst, it would be another airplay smash that peaks Top 10. The iTunes performance when it was treated as a buzz single, which was released second while the first was already gaining huge momentum, is irrelevant.
"Legendary Lovers" could go either way. Radio people think it sounds weird, too, so again, it might need a remix. I could see it being massive or flopping, but I don't think it would be another medium sized hit like "Unconditionally" and "Birthday."
But there's really no point in debating this anymore until there's confirmation that a consensus has been reached in favor of releasing another single from Prism in the fourth quarter.
Katy still had some momentum after B'Day (DH was still going strong). Even now after TIHWD's "underperformance" she can smash with the right single choice and promotion. 
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nice spins though 
