If what you want is a dps magic user then you can't go wrong with a mage.
The priests dps magic build is the shadow priest, shadow priests have good magic damage but their true value is in utility and versatility.
As a priest, a priest of any build, from time to time you will be expected to heal.
If your not into healing, or not into doing things that are not allways going to be damage, then maybe the mage would be the better chose of the two.
Edited, May 5th 2007 10:22pm by Maxzzzz