On my 05 America, I have the tall one which is noticeably more comfortable for my wife. I also replaced my original pillion with a 07 one which is larger and a LOT more comfortable. The larger seat also sits up a bit higher in the back which she likes as well.
As Richard mentioned, the taller one can better handle attached luggage. When I travel alone, I use a backrest bag between me and the sissy bar AND I put a T-Bag on the rack behind the bar. The combo bar/rack is very useful.
