You want option B.
Option (A) has the cons:
- You lose 2 additional disks to parity using 3 RAID groups
- You lose an additional disk as a hot spare
- When you add an additional shelf you will have 12 spare disks to decide what to do with
- Bottom line you have less space with this option
Option (B)
- No cons I can think of
- Pro -> More space than option (A)
- Pro -> More data disks in the aggregate thus giving you slightly more IOPS
- Easily add another shelf and use 23 disks with 3 hot spares