The fact that you have 15 disks in a clariion dae, you would leave one for hotspare.
You could either do a 12+2 DP RG or a 6+2 and a 4+2. The 6+2 and the 4+2 arent really cost effective, if you are using anything less than 1TB drives.

The reason netapp pushed raid 6 for so long is because they wanted ppl to use the entire dae in one raid group, there by claiming a higher utilization. but the higher utilization came at a performance cost.