Hi Tibor,
These points should clarify your doubts:
1. Transactions are not cached but queries are.
2. Advantage of using Xacute query is that the Illuminator service used to call Xacute query supports JSON from MII 14.0. Use content-type as text/json in the URL. But the restriction is that the transaction should return the XML in MII Rowsets-Rowset format.
3. Though both the calls would take almost time(I think) but you should call transaction directly( no need to wrap it with Xacute query if not required).
Regards,
Rohit Negi.