technically, he is right though- if you want to be pedantic- since the OP asked which is better to extend their desktop; since they both can it stands to reason that the cheaper alternative is preferable.
However, on the other hand, that is being just that- pedantic- nobody asks "which is better" in reference to a duplicate feature between two cards; instead one must fill in the blanks. One can thus assume that they want to know which gives better performance when extending the windows desktop on a second monitor. This is not the same question- They will be the same with 2D graphics, however, Windows Aero (if it employed) will almost assuredly benefit from the more powerful GPU, especially when using two screens. This of course extends to any other OS when running games that support using two monitors (very few do, that I know of).