The Cloud dome is the taller of the 2 domes at Gardens by the bay. However, do not be fooled, there's just as much ground to cover. I prefer this dome to the flower dome because it's a more structured walk.
After you enter and behold the amazing 4/5 storey tall waterfall, you enter the 'mountain' and take a lift up to the top, from there the path takes you through gardens, paths that meander outside so you can view the green walls, a crystal forest and at the bottom, a simulation of what -5degrees would do to the earth, before exiting through another garden. The content of the cloud forest dome is more varied, more interesting, and, in my opinion, better presented.
If you only have time for one dome, make it the Cloud dome.
Next: The Skywalk