Some of this hinges on the hardware involved. I prefer RAID 1+0 for simplicity and rebuild times. It's tough to give a generic answer without more details, though...
Things to consider:
The disks installed in the system: SAS? SATA? Nearline SAS? This impacts the failure rate and failure mode, as well as array rebuild times.
The anticipated use for storage: Your performance requirements may drive the design. Random I/O? Sequential? Read-biased? Write-biased?
Interconnects: How will the storage array be connected to the server? SAS? Will you be using a single connection to an HBA? Two? Multipath? 3Gbps? 6Gbps? There will be a ceiling in storage throughput because of SAS oversubscription. So this factors into the design because of that performance cap.
Controller: I always come from an HP SmartArray perspective, but I suppose the rest of the world uses LSI and PERC controllers. This may be a moot discussion, as LSI controllers can't have more than 16 disks in a single-level virtual drive; e.g. you wouldn't be able to create a 24-disk RAID6 volume. You can do this with HP controllers, though.
Resiliency: Do you plan to have online spares? When you consider a nested RAID level like 60, that becomes important.
So, assuming a controller capable of both. Your options are really 4 x 6-disk RAID6+0, 3 x 8-disk RAID6+0, 2 x 12-disk RAID6+0 and a 24-disk RAID6.
Determine the space needs, as they vary. Then evaluate the sequential performance capabilities of each. I'd suggest 3 x 8-disk as a reasonable if you go nested and aren't interested in RAID 1+0.