Well...Yes 91 is pretty much the lowest quality fuel, but whether or not you'll get better mileage depends on the type of car you have, because high octane fuel goes to waste in cars with low compression ratios.

You say you have a 2014 Mazda CX-5, as far as I can tell most of the variants of the 2014 CX-5 have a compression ratio of 13:1. That's fairly high, average sedans are 9:1 or 10:1.

For those who don't know, the compression ratio is represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity. So in general higher compression ratios mean higher performance in the engine (obviously not the only factor but the main one). https://en.wikipedia.org/wiki/Compression_ratio

So in your case it is probably best to be using higher octane fuel because using lower octane fuel in high compression ratio engines causes engine knock, which is what people are talking about when they say you will end up having higher maintenance costs. https://en.wikipedia.org/wiki/Engine_knocking