you sound like you're confusing sftp and ftp to some extent. whilst sftp supports a similar user command set to ftp, that's all, it is NOT ftp.
Additionally sftp is a subsystem of ssh, so you wouldn't look to separate them normally. It's possible to look to run two separate instances of sshd on different ports, disabling sftp on one of them, this isn't going to benefit you realistically.
I'd imagine that an ftp user has a dedicated account for that function, so I would look to make that users shell a non-login one, using, for example, scponly - http://www.sublimation.org/scponly/w....php/Main_Page
Here, there is no benefit in looking at port numbers at all.