In your opinion, which one is better in IVs?

You will never see the difference in performance between them.... IVs are the single most overrated thing in Pokémon Go

Not my opinion, just a fact: 91%>>84%.

If you are looking for specific match ups, then I recommend you to use the breakpoint and bulk point calculators on this website and check if the difference in attack, etc makes a difference in performance (i.e. damage and durability).

Both are bad

Depends. As a generalist, the 91% is better as it has more bulk to it. For raw dps the 84% has higher theoretical dps but that can change from raid to raid based on breakpoints/bulkpoints.

In practice each is within 1-2 levels of hitting the same break/bulk points as the other... so you realistically probably won’t notice any difference between them.

When Palkia is released, 15atk on Rayquaza is a must have to reach the last breakpoint when battling with a best friend and without weather boost. However, this is the only case (for all (pseudo) legendary dragons from gen 1-4) that are weak to dragon) in which its max dps is higher than one with 12atk: Dragonite, Salamence, Rayquaza, Garchomp, Giratina A, and Giratina O all have their last neutral weather best friend breakpoint plenty below 40 with 15atk which is thus reachable by a Rayquaza with 12atk too.

I chose best friend and neutral weather as that's the most common combination for me. You can ofcourse play around with breakpoint calculators yourself