Re UD vs. LD:

The primary differences I've noticed are: 1) Cabinets next to window seats on Upper Deck mean tons more storage space and a place to put drinks, trays, books, newspapers, etc. 2) Higher chance of an empty seat next to you on the lower deck if flight not full, as Upper Deck seems to fill up faster than lower deck. So... weighing your chances of an empty seat vs. guaranteed cabinet space.

One other thing, and this may be unique to me. If I'm seated on the UD, I feel perfectly comfortable walking around the whole plane (other than F cabin) when I want to stretch my legs. The area at the top of the stairs is a good space for stetching/fighting DVT. But if I'm on the Lower Deck, even in First Class, I kinda feel like it's not kosher to go to the Upper Deck on a walk, 'cause UD is sorta just for UD pax. So, Business UD means more walking-around area to me than Business LD.

I don't think it's that KE doesn't sell those middle seats, I think it's that they're just highly likely to be the last ones assigned since they're essentially the least desirable. So if four Business seats go unsold on a 744, in all likelihood it'll be those middle seats. That's why getting an aisle seat in the middle section of the LD gives you a decent shot at having the seat next to you empty.