Which one is the best?

The 15 attack one. Since, at level 40, a HP IV of 15 gives the same health as a HP IV of 14 for Rayquaza you're basically trading 1 defence for 1 attack.

You need to clarify the stats. You only give 2 stats for each and put one stat between Attack and Def so not even clear which it's supposed to be and which stat is missing. I would wait until you have a specific raid that you would use it for and then take it to the break point that benefits your purpose, for some raids it may be better to power up both to a break point as opposed to maxing one and leaving the other low.

There is no substantive difference between the two.

Essentially at Level 40, the IV 98 lasts 1 more Fast Move than IV 96, but IV 96 might take down the opponent with 1 less Fast Move. If you aren't going up to Level 40, it's possible that there is literally no substantive difference between the two.

Given that, I'd go with the IV 96 just 'cuz I like the higher Attack. But the other Rayquaza would be a long term project to raise up as high as possible.