Choice of NAT64 prefix

The advantage of using your own prefix is that you can route it over the internet. That way users outside your own network can use it. Whether that is a feature depends on what you plan to do 🙂

One advantage of using a different prefix for each NAT64 is it's possible to gracefully "drain" a NAT64 for maintenance.

NAT64 (like NAT44) is a stateful process, so if you have two separate NAT64s serving the same prefix and you re-route client traffic from one to the other you will immediately break any active connections.

OTOH if your NAT64s use different prefixes and you use DNS to direct client traffic you can move new connections to a different NAT64 while allowing existing connections to complete on the existing NAT64, by waiting a while between switching the DNS and taking the NAT64 down for maintenance you can reduce the number of active connections you break.