As somebody who likes it I think I would explain it along the lines of FreeBSD vs Linux.
AIX is a complete package OS with some nice enterprise features, and it is modern and similar enough to make porting from Linux not too terribly difficult. Even something like RHEL is still very much more in the Linux tradition of lots of pieces of independently developed software kinda thrown together and called an OS, at times haphazardly.
That being said, we of course run RHEL as well, but mostly for cost reasons for less important applications, as opposed to any particular dislike for AIX.
USS on zOS is a much more difficult IBM UNIX to work with in my opinion, and tends to generate stronger opinions from both sides of the proprietary/OSS fence. They are making progress, but the compiler and compatibility situation is not as good as POWER/AIX.