Depends on what you want to do.  If you fit to page, the print is fit inside the printable area.  If you fit to paper, it'll fit it to the paper size and the non-printable margins will just not print.  The latter is used primarily for reproducing scans (8.5x11 to 8.5x11, A4 to A4, etc.) or when you design your own layout and pick the paper size (like A4) and you need it to print at the designed size (knowing that the non-printable margin area can't be printed).

Example, you scan an 8.5x11 page and you want a "copy" of it.  If you use fit-to-page, you'll end up with a reduction because your 8.5x11 scan will be shrunk into an 8.266 x 10.766 area (Epson example).  If you use fit-to-paper, you'll get an exact copy.  The only difference would be if there is something at the edges of the paper (beyond the 8.266 x 10.766 printable area): that just won't print.

Regards,
Mike