I went through this about 2 years ago when I gutted and renovated a 1/2 story, including the addition of a large dormer. Part of my issue was that I could only get about 5" of insulation between the rafters, and lowering the ceiling height was not an option. To make a long story short, after months of reading and getting advice on these forums, I went with an unvented/conditioned attic using open cell spray foam. My main concern was the radiant heat, and this certainly performs much, much better than the vented/fiberglass setup that I removed. It is still hotter in the half story than the rest of the house, but it is very livable.
I do wonder what the difference would be if I had the room to do a vent space/radiant barrier/foam setup, but I don't regret going unvented given what I had to work with.
EDIT: Good to see a pro recommend this setup right above me. ; )