The Serial.print routines send data out the Due's Programming USB
Port, and one can use the "Serial Monitor" Tool to see that output.
The difficulty here is getting this Serial Monitor window
active in time to catch the first output from the program.
In use, it appears that opening the Serial Monitor window
causes the uP to reset, starting the program over again.
Not a nice "feature" if one is trying to control some
real time process. But, handy to see the first debugging
output (from the Setup() routine.
The SerialUSB.print routines send data out the Native USB Port,
and one would use some program in the PC to "catch" the data.
One might have a custom program, or use some "dumb terminal"
program to have simple interactions with the Due.
Here, the Virtual Comm Port appears to get reset whenever the
Due gets reset, so it is difficult for a program to open the Comm
Port until a while AFTER the Due program starts running.
But, you can start the Due program running, then the virtual
Comm Port will (re) appear, the PC program can start, open the
Comm Port, and start reading the incoming data, only having
lost the initial data from the Due.
Not convenient, but that appears to be the way it works.
Otherwise, it can appear to you that no data is coming to the PC.
Cheers, Gary