the performance is going to be the same - assuming the same data.

The precision and scale are simply "edits" - constraints on your data. They do not affect how the data is stored on disk, only what values may be placed in there.

So - that said, you would use the precision and scale as a data integrity constraint. If you have "money" that you are storing, you might use:

ops$tkyte%ORA11GR2> set numformat 999999999999.99

ops$tkyte%ORA11GR2> create table t ( x number(*,2) );
Table created.


ops$tkyte%ORA11GR2> insert into t values ( 123432432.99 );
1 row created.

ops$tkyte%ORA11GR2> insert into t values ( 123432432.9999 );
1 row created.

ops$tkyte%ORA11GR2> select * from t;

               X
----------------
    123432432.99
    123432433.00

please do take note of the rounding that takes place! If that is not desirable - you should:

ops$tkyte%ORA11GR2> create table t ( x number constraint x_must_be_money check(x=round(x,2)) );

Table created.

ops$tkyte%ORA11GR2> 
ops$tkyte%ORA11GR2> insert into t values ( 123432432.99 );

1 row created.

ops$tkyte%ORA11GR2> insert into t values ( 123432432.9999 );
insert into t values ( 123432432.9999 )
*
ERROR at line 1:
ORA-02290: check constraint (OPS$TKYTE.X_MUST_BE_MONEY) violated


ops$tkyte%ORA11GR2> select * from t;

               X
----------------
    123432432.99

using number(x,2) will just ensure that all data inserted in there has at most 2 decimals places - it an edit.

And the edit will not affect performance at all - since 1111.22 stored in a number, number(*,2), number(6,2) would all be stored precisely the same:

ops$tkyte%ORA11GR2> create table t ( x number, y number(*,2), z number(6,2) );

Table created.

ops$tkyte%ORA11GR2> insert into t values ( 1111.22, 1111.22, 1111.22 );

1 row created.

ops$tkyte%ORA11GR2> select x,dump(x,16), y,dump(y,16), z,dump(z,16) from t;

               X
----------------
DUMP(X,16)
-------------------------------------------------------------------------------
               Y
----------------
DUMP(Y,16)
-------------------------------------------------------------------------------
               Z
----------------
DUMP(Z,16)
-------------------------------------------------------------------------------
         1111.22
Typ=2 Len=4: c2,c,c,17
         1111.22
Typ=2 Len=4: c2,c,c,17
         1111.22
Typ=2 Len=4: c2,c,c,17