I use 3 x7 drive RAIDz3, i use 7 4Tb hdds, 7 2Tb hdds, and then another 7 2Tb hdds. The 4Tb drives are all RED's (not pro's), and all the 2's are RE4 drives, i feel RAIDz3 for me is best because im theory i could loose 1 drive, and then during a resliver loose another drive or 2....hopefully not as i run both short and long SMART tests on a schedule, and i use the lovely scripts from the forums to get the reports once a week and jazz.
I like the mini, but i feel it doesnt offer enough for what *i* personally needed, so i went with a 24 bay supermicro :p .
RAIDz2 is the RAID10 equivalent.
Check out the calculator here before you decide on what size drives/how many
https://jsfiddle.net/Biduleohm/hfqdpbLm/10/embedded/result/
I also use Crashplan Pro to backup my entire server, when i say ENTIRE i mean all of it....i have 27Tb in the cloud atm with crashplan, some of it is archived and some of it is active. But i also selected crashplan to never delete it when deleted off server/drive..so in theory i have stuff from the first backup of crashplan to now