Ken, if budget allows only one, I'd go with the hand-held light: much more versatility. But if budget allows both, having both is the best choice in my opinion.
The gun-mounted light points a loaded gun at anything you directly illuminate. Many "bumps in the night" involve family members or houseguests, which can be hugely psychologically traumatic for the person "lit up," even if you are the perfect, mistake-free human who can always "keep the booger hooker off the bang switch." Hence, the backup hand-held unit for search purposes.
However, in the rare circumstances where you will need to fire your weapon in the dark, nothing beats gun-mounted white light. A combined light/laser sight module may give you slightly more speed/accuracy under poor lighting conditions.