To show a popup, we can use a dialog (div) or a window, which way will be better, pros and cons ?
Most users have popups disabled and most find popup windows annoying just because they could open it in a new window if they please. They don't like the site doing it for them.
A dialog div would be best because it doesn't open a new page and most users are accustom to it
Well, you overlooked the right solution, which is: don't do it.
May I suggest - the use of "Highslide" for a sorta pop up (easily found with a search) ......... I have found this to be an excellent approach to the need for pop ups within a window and so, space saving too. It is useful for images and text equally.
On this page - http://www.thingameez.com/ - there are two cartoons using it and also the "Remind Yourself about the 2a" which opens a text box.