I think a lot of it is personal preference but I prefer the 400 with the single seat in the middle - are you flying alone? You might want that seat for aisle access with no one bothering you. The 300 has backwards/forwards seats. Although some FT folks have said they prefer to sleep in the backwards seats because of the upward angle of the seat in flight, the privacy sucks as you are staring into the faces of all the people next to you facing the opposite direction. Bad design in my book. Which would be fixed if they would just align all the seats so they line up - ie a row of all forward facing and then a row of all backwards facing.