The only thing flac has over aiff is smaller file size, and I am not interested in that, as storage is inexpensive.
I am not concerned about which one is more widely used by other end users. AIFF suits my use case.
AIFF is an industry standard that is well maintained, because it does have its uses.
They are both the same performance-wise. The thing I don't like about AIFF is it was developed by Apple for their operating systems. I am not an Apple user and don't ever plan to be. I'm a PC and Android user. I can play AIFF on my PC but always convert them to FLAC if I keep it.
FLAC on the other hand was not developed by any for-profit company and was intended to be a free software for all to use hence the name "Free Lossless Audio Codec". FLAC has two big advantages over AIFF and that is the smaller file size without any sonic loss. That is a significant advantage, especially for someone who has a massive library (like me). The other advantage is it is 100% compatible with non-Apple devices and systems.
The final thing is FLAC is far more widely used nowadays than AIFF is. If you are going to pick one and are not an Apple user, it's pretty-much a no-brainer.