Hi,
please confirm: you are talking about external RTC chips.
If so, each of them thas it's own qualities. You have to determine which quality is important for you.
* cost
* size
* external part count and their cost
* accuracy
* supply voltage range
* current consumption
* data format
* interface
* access time
* internal NV memory
* watchdog
* alarm
* automatic battery switch over
* low battery (time information) failure detection
* and a lot more
No one of us can tell you what feature is more or less important for your application. It's your job.
Klaus