Can't decide between two QNAP models + some ZFS questions?

[ed]

With limitations I mean, that you can not add more disks and not as many apps would be available for this OS. But otherwise, it has some great enterprise-class features, but also you may need more RAM for this (built-in RAM size should be OK at the start).The cache is something you can add later, no need to do it at the beginning.I hope this helps.