I would like to connect to a data source running on Oracle. Should I use SID or Service Name?

Answer:
Xpert BI can use SID or Service Name when connecting to Oracle database server. Oracle recommends remote connections via the Service Name. For non-cluster/standalone Oracle instances SID and Service Name often have the same value. For Exadata/Oracle RAC, the Service Name is an alias for all SIDs participating in the cluster. 

For Import Source Metadata you can connect directly to one clusternode using the SID.  Failover, load-balancing and so on will not be used but that does not matter when retrieving metadata. 

https://community.oracle.com/thread/2305499?tstart=0