which one to use? Temp Table or Table variable

Well , IT depends. Based on the uses and limitations of both the areas and purpose of use may differ.

Watch for more space.

Configuring AlwaysOn-High Availability

Refer http://nirav.extreme-advice.com/2014/06/04/configure-alwayson-high-availability/ to learn about configuring Always On feature.

Non Clustered Index on HEAP ( A Table without a Clustered Index)

SQL Server allows us to create Non Clustered Index(es) on HEAP- A Table without Clustered Index.
A Heap doesn't have a Clustered Key, the SQL Server maintains a unique identifier for each row which is called as RID (Row Identifier) and maintains this RID along with Non Clustered Index