The Freescale D-Bug12 monitor communicates with a PC via a simple ASCII based serial interface, so it only requires a terminal program, like AsmIDE, MiniIDE or Hyperterm, installed on a PC to communicate with the D-Bug12 monitor. You can enter an ASCII command, such as DM (display memory) or MM (modify memory), and it will be recognized by the D-Bug12 monitor. If you have worked on 68HC11 programming before, you would be familiar with the BUFFALO monitor. The D-Bug12 monitor for the HCS12 works in a way like the BUFFALO monitor for the 68HC11.

The Freescale serial monitor communicates with a PC via a proprietary binary code based serial interface, so it requires a special program, such as Code Warrior installed on a PC to communicate with the serial monitor. If you enter an ASCII command it will be translated by the Code Warrior to a binary code that will be recognized by the serial monitor. An ASCII command entered in a terminal program will not be recognized by the serial monitor.

The advantage of using Code Warrior is that it includes a C source level debugger which allows you single step through your code in assembly or C language. In order to use this feature you need the Dragon12 Plus board to be installed with the serial monitor.

The advantage of using a terminal program with the D-Bug12 monitor is that it's easier to use than the Code Warrior with the serial monitor. If you are new to Code Warrior it will require a considerable amount of time to learn how to configure and use the Code Warrior.

You probably should order the board pre-loaded with Freescale D-Bug12 monitor:

If your professor teaches your course using the D-Bug12 monitor.
If you bought Prof. Tom Almy's textbook and want to learn HCS12 programming with his book
If you do not use Code Warrior's debugger, but only use Code Warrior to create and assemble or compile source code using the assembler or C compiler included in Code Warrior to generate an s19 hex file and finally use a terminal program to download the s19 file into RAM , run it from RAM. In this case, you need the board installed with the D-Bug12 monitor, not the serial monitor. You debug your C program by inserting "printf" statements within your program.
If you use EmbeddedGNU or Imagecraft C compiler to create and compile source code to generate an s19 hex file, then use Imagecraft IDE or EmbeddedGNU to download the s19 file into RAM and run it from RAM. In this case, you need the board installed with the D-Bug12 monitor, not the serial monitor. You debug your C program by inserting "printf" statements within your program.
If your programs is very small. It's not worth spending time to learn the Code Warrior for blinking a few Christmas lights. The D-Bug12 monitor will be good enough for you.
If you only use a HCS12 board occasionally for small projects. It's not worth spending time to learn the Code Warrior and you will probably forget how to use the Code Warrior in a few months anyway if you don't keep using it continuously.
If you are new to the Microcontroller programming field. It's easy to start with the D-Bug12 monitor.

You probably should order the board pre-loaded with Freescale serial monitor:

If your professor teaches your course using the serial monitor.
If you bought Prof. Richard Haskel's textbook and want to learn HCS12 programming with his book
If you want to learn how to use a modern professional IDE and prepare yourself for working in the industry.
If you want to use the Code Warrior's True-Time debugger for debugging your code. You want to be able to single step through your C program in debugging.
If you want to use a HCS12 board as a port of MATLAB and Simulink with Code Warrior IDE in Mechatronic.
If your program is complex and debugging your code will be easier with Code Warrior.
After school if you are going to use a HCS12 board continuously for some large projects.