Postgres insert performance and storage requirement compared to Oracle

Hello Experts,
My application uses Oracle DB, and makes use of OCI interface.
I have been able to develop similar interface using postgreSQL library.
However, I have done some tests but results for PostgreSQL have not been
encouraging for a few of them.

My questions/scenarios are:

1. How does PostgreSQL perform when inserting data into an indexed (type: btree)
table? Is it true that as you add the indexes on a table, the performance
deteriorates significantly whereas Oracle does not show that much performance
decrease. I have tried almost all postgreSQL performance tips available. I want
to have very good "insert" performance (with indexes), "select" performance is
not that important at this point of time.

2. What are the average storage requirements of postgres compared to Oracle? I
inserted upto 1 million records. The storage requirement of postgreSQL is almost
double than that of Oracle.

Thanks in anticipation.