FCalias is used for zoning only.

FCalias can contain multiple PWWNs

FCalias is configured per VSAN

FCalias is propagated via distributing the full zoneset.

Device alias is 1 PWWN

Device alias can be use for multiple functions like port security, IVR, and zoning.

Device alias is not VSAN specific

Device alias is distributed via CFS (Cisco Fabric Service)

Best Practice is to use Device Alias if the fabric is made up only Cisco MDS.

Hope this helps,

Mike