The commands are different. The 1st makes a backup of the controlfile in an RMAN backupset. It will use the default %U format.
backup current controlfile;
The next is making a binary backup copy of the controlfile to disk.
backup current controlfile to '/path/filename.cf';
This copy can be used without restore to mount and restore an Oracle database from an RMAN backupset. If in nocatalog mode it essential to make this copy last after the backup of the database and the archived logs. Oracle9i handles this with the autobackup controlfile feature.