Guid in WP_Post table

I had a look in phpmyadmin and saw this particular column called guid under the wp_post table. Some of my post are using ?p=39 and some are using pretty permalinks like 2007/09/19/test/. I have a mixture of this in my table, so which is the one I should be following? All my old posts are classified under the ?p=39 format while the new posts which I made with WordPress 2.3 are following the 2nd one.

Any assistance would be much appreciated.