The glass slide is totally overrated, no speed or height at all. But the views and the glass floor themselves are great, recommend to visit at evening (dark) hours.
The observation deck is very well located - next to a large number of skyscrapers and the main skyscraper of the world, which offers a great view. The price no longer includes the descent from the slide - you need to pay an additional 25 dirhams. I do not recommend it - it is too short entertainment. But there is free underground Parking - at the entrance you need to say that you are going to Sky views.