It depends entirely upon what you want to accomplish. For example, if you have a single word underlined in mtext, it frequently throws AutoCAD for a loop and causes all of the mtext to be underlined. Further, if you want to have different fonts for specific items(i.e. a heading) if you include it in the mtext object, it will most often take on the properties of the rest of the mtext object. While it is possible to edit the mtext in a regular text editor (like notepad) and enter the proper RTF codes to make the text behave properly, the implementation of the MTEXT editor in AutoCAD makes many assumptions and can cause the text to be displayed in a manner not consistent with your needs or desires. Mtext also does not support the ability to manually adjust the width of the font. In dtext if a piece of text does not fit properly, you can adjust the font width up or down to get the desired results. In mtext, you adjust the size of the mtext box, but this can cause text to not be "fitted" to the space required. I use both styles of text and each has their place.