Options generally include: fixed ladders, stairs, and movable ladders.
The vertical ladder does not take up much space, but is a poor choice for the elderly, small children, or those with bad joints.
As for movable ladders, they are best for freeing up extra space when you need it. For example, if there is a wardrobe underneath, moving the ladder allows for easy access into the wardrobe.
Stairs take up a lot of space, but are generally more safe, as you can hold the rails as you ascend. Having a staircase can also create extra storage space.