Basically timed means you have to book a time a day or so in the future - immediate means you get in that day. We did timed and it was good - went in at alloted time and no problems. We weren't rushed or anything to finish the experience either.
If you have visitors coming then I recommend you book timed as soon as you know their dates
I want to take a visitor to the observation desk and when I went to reserve tix online, they had two types:
Timed for 100AED
Immediate Access for 400AED
What was your experience with the "timed" tickets?