There is probably no advantage. Both are digital formats and there should be no image quality difference between those two.
The only reason two switch are:
- If you have a monitor with speakers and you are now using both a DVI cable and an audio cable. (thus using less cables)
- If you have multiple display port monitors and you want to chain them, so you use less cables. (Unlikely with the current hardware, but slowly becoming more common).
- If you want to connect two monitors, and the other monitor has no display port.
- If you have a single link DVI cable and a monitor which can handle higher resolutions that the single link cable supports. (Dual link DVI cables would also solve this).
From the OP I think only the first option could be relevant.