The IAP has the added feature of the self-managing Instant OS.
We've stopped buying the non-Instant units, and convert where we use controllers and not where we don't.
The principal benefit being that I can keep one spare to replace any failed unit rather than one of each.
The side benefit is that I see some of our controller based installations going Instant in the future and all I'll have to do is convert back and set a few bits in the VC and I'm done - vs. physically replacing all of them.
Only downside is you have to have a controlelr license for the converted IAP and you have to power up the IAP and click a few bits to convert it - not that bad.