Main difference between the two is the version of Android you would get. Dell Venue 8 would have complete Android experience while Kindle Fire 8.9 will be a heavily molested version of Android which don't have access to Google Play.
Apart from that KF 8.9 have beautiful 1080p 8.9" screen.
Btw, what will be your primary source of "HD" content? If you are somewhere where Internet connection is good and Amazon services your preferred way to get content.. then KF 8.9 is good. But if you are in India, and want to copy the movies over to the tab and then view it.. then DV8 or Nexus 10.
Nexus 10 throws out KF 8.9 out of the water.