Better ZFS choices for 24 disk pool - more vdev or higher raidz level

I have the following conundrum.

I have 18x 12TB disks and (maybe) 12x 20TB disks.

I've come up with the following options;

1. A pool consisting of two vdevs. Each vdev is 12 disks with raidz2, so I get 320 TB of raw capacity. 2. A pool of 4 vdevs. Two of the vdevs are 6x 12TB and the other are 6x 20TB. Each vdev is raidz1. Same overall capacity as option 1 - 320 TB. 3. A pool of 4 vdevs, as previous, but only one vdev is 20 TB disks. Capacity is 280 GB 4. A pool of 3 vdevs, all 12TB disks. Capacity is 180 GB

Which is preferable, and why?

(I realise the larger capacity disks are probably more desireable, but I may not have them so I'm looking for a more architecture based answer, rather than **mooooaaarr disks**!)

Thanks for your collective wisdom!