Hi umeshunited,
In terms of functionality, and how ASA is using this information - nothing. Both commands will override hardware MAC address, and use this given one.
If I remember correctly, as of certain version of ASA code (I'm not sure exactly which one, maybe 9.5), if "failover mac" is used, you'll get a warning that this command will soon be deprecated, and that you should move to interface MAC command.
Either way, I would recommend using one or another option (I personally use one under interface, for some time now), as it is quite handy in case your primary unit fails, it can make your life bit easier ;)
Best regards