they are really 2 different products for 2 different purposes.
Image resizer allows you to dynamically resize an image on the server when it is loaded into a web page, this allows you to have one main image that can be displayed in multiple sizes.
Digital File Pro allows you to Uplaod a file from the client machine to the server, on upload you can specify resize settings, or upload different copies of the image.
you can also delete and download files using Digital File pro.