Both a single height and multiple height roost will work...
Multiple height roost allow for natural pecking order to take place, the dominate birds will generally roost on the higher roost, they also generally allow the birds to 'hop' vs 'fly' to a roosting location (or hop up to a higher location) this is important for some heavy breeds that don't have the ability to fly well enough to get to a higher roost... The reason they are generally 'laddered' at an angle is so that the upper birds don't poop on the lower birds all night, and it allows the hoppers to climb to a higher roost easier... Also ladder roost generally allow you to put more linear feet of roost into an similar sized area, of course coop design will play a large part in your ability to install either type of roost...