For most purposes there is no difference. There are a few cases where it does matter though.

In the "must be is corner", we have:

Explicit cursors

declare
  cursor c as select * from dual;
begin
  null;
end;
/ 

PLS-00103: Encountered the symbol "AS" when expecting one of the following:

PL/SQL array types

SQL> declare
  2    type typ as table of pls_integer;
  3  begin
  4    null;
  5  end;
  6  /
  type typ as table of pls_integer;
              *
ERROR at line 2:
ORA-06550: line 2, column 15:
PLS-00103: Encountered the symbol "TABLE" when expecting one of the following:
object opaque

Though this only applies in PL/SQL. SQL nested tables can use either:

SQL> create or replace type typ as object (x int);
  2  /

Type created.

SQL> create or replace type typ_arr as table of typ;
  2  /

Type created.

SQL> create or replace type typ_arr2 is table of typ;
  2  /

Type created.

-----

And in the "must be as" corner we have:

Create table as select

It's implied in the name - you can't "create table is select"!

SQL> create table t is
  2    select rownum x from dual;
create table t is
               *
ERROR at line 1:
ORA-00922: missing or invalid option

Materialized Views

SQL> create materialized view mv is
  2    select 1 n from dual;
create materialized view mv is
                            *
ERROR at line 1:
ORA-00905: missing keyword

Standard Views

SQL> create or replace view v is
  2    select 1 n from dual;
create or replace view v is
                         *
ERROR at line 1:
ORA-00905: missing keyword

Directories

SQL> create or replace directory dir is '/tmp';
create or replace directory dir is '/tmp'
                                *
ERROR at line 1:
ORA-00905: missing keyword

PS - thanks for using LiveSQL!