So just did two A380 back to back flights in J - 9K vrs 23K and thought would add to my prior comments here:
9K has all the benefits listed above, up front, window, extra leg room than isle etc.
However actually having flown 23k - than looked at it - comparing it to 9K:
Cons:
23K is a pain to get out of - most other seats in J, have something you can grab hold of to sit up from being fully flat to go to the bar etc mid flight - but in 23K, there is no such anything to grab hold of.
It is further down the plane - so you are one of the last J pax to deplane
Pros:
23K has the most leg room of any J seat I have sat in on EK A380 - I could stretch out and wasn't limited at all in length - as even the likes of 9k, if I stretch when in flat position - my toes and head touch both ends.
23K is a nice secluded seat, that almost know one really is going to see you. If you want the most private seats in J - 23A/K would be a good choice
23K is nice and close to the loos and bar - but again note it isn't the easiest seat to get out of in the cons.
23K - but it seems to be all the seats in the mini rear area that is window - the cupboards between the seat and the windows are bigger. I have a rather big, 17" laptop (is also 6kg) - and in all the front cabin in J, I have to put it sideways in them in flight to close the lid, as it sticks out tons - but in 23K, I could put it in longways with room. The cupboards simply have roughly an extra 20-30cm in extra depth on them.
On a side note - while in 9K, SIN-MEL, I got 8J's food tray dumped all over me. FA - in their quest to balance 2-4 trays to walk them to the rear, in balancing the tray from 8J to get my tray in 9K, slipped and tipped it all over me, half drunk water and wine and all, broken wine glasses, and overall mess. While I had my own PJ's as well as change of clothes - they offered me a pair of F PJ's and to handwash and dry my clothes.