Well...I didn't really figure anything out. I just went with what I knew. Thanks for the comment though. I did some research to understand vDevs better.
My main NAS is used for media and backups of the family's various computers. I have a second older "archive" system that I sync to every couple of weeks. All the really critical stuff, family pictures and movies and such, is backed up to dropbox. The dropbox folder is synced to my notebook, my desktop at work and the NAS device. Since I don't do real time syncing between the production and archive NAS devices I've got some time to recover any dropbox files that are accidentally deleted.
The new system is going to end up at work. Once I get all my data copied over to it, I'm going add more drives to my old system so that it matches the new one. After that, I'm going to add more capacity to the archive NAS. I've got a bunch of older 2tb drives. When I'm done I should have three copies of everything. One at work, a production one at home and a backup archive system also at home.
Having two RaidZ2 vDevs or or a large RaidZ3 vDev would have increased reliability but all in all, I think I've got things backed up pretty well. I figured I could trade some safety for extra space.