While you can't run much in the way of custom resolutions with the adapter (I do have 856x480p working), custom timings do work with Powerstrip.
I can get these resolutions to work @540p/1080i timings with the purple adapter & 9500pro w/ CAT 3.2 drivers, displayed on a Mits 55819:
640x480p
720x480p
856x480p
1776x1000i
1920x1080i
All progressive scan timings are locked at 33.75KHz and 59.947Hz, and interlaced are 33.75KHz and 30Hz. Check the guide in my sig for more adapter info.
If you only need 2 resolutions, desktop & DVD playback, then the dongle is recommended...I use 856x480p and 1920x1080i.
If you need for more resolutions for gaming, etc, go with a transcoder.
The advantage of the dongle is price, ease of use, and 1080i DVD playback.
The advantage of a transcoder is more control and range of flexibility of custom T & Rs-when connected to the VGA port-...and no MV restrictions.