depends on what you are doing. but you should do one or the other: either use builtin pullup/down, or use Port.ResistorMode.None and use external ones.
The ones activated by Port.ResistorMode are within the chip itself, and are nominally 50K, I believe, for the st32f4. Whether that is useful to you or not depends on what you are doing specifically. The feature is there as a convenience to circuit designers, to avoid having to populate a bunch of resistors on the board -- especially for things like buttons, etc.
EDIT: hanzibal beat me to the post while I was composing, but I am so glad, because he stated that the resistors are applied 'infernally', which so often is true!Go to the full post