Portrait, simply because the sluggish omnibar/keyboard animation isn't as noticeable or bad. In landscape orientation tapping the omnibar while currently on a site reveals the sluggish animation as the address highlights, slides left, and the keyboard comes up from the bottom ... and it's all down to the stupid Yosemite style Favourites/Bookmarks box that comes up loaded with semi transparency.
I'd happily use Reduce Transparencies via Accessibility because it stops this "issue" but the dock looks ugly as sin and it adds (utterly unnecessary) black text for the apps in the dock rather than white. Bizarre design choice.