Both have their advantages and disadvantages.

Treadmills mostly work the hip flexors, knee extensors (quads) and dorsi flexors of the ankle because the task is to get the leg that is back, forward before it goes off the mat. Thus it is not equivalent to walking on the ground, where the task is to extend the hip while the foot is planted to propel the body forward. That works the hip extensors, knee flexors and plantar extensors of the ankle.
Having foot drop where there is a problem with dorsi flexion might make using a treadmill difficult and potentially dangerous.

Elliptical trainers work both the the hip flexors & extensors, quads and calf muscles because the task is to push downward on the leg that is up. Elliptical trainers also stabilize the lower leg and foot, so actually lifting it off of the machine is not necessary as it is on a treadmill. Thus an elliptical trainer might be best for those with foot drop.

Both machines also work the trunk, so the abdominals and back, as well as the upper body if one uses the arm attachments.

You also might try a cross country ski machine, which works all of the main prime movers of the lower and upper body, as well as the trunk. It also doesn't require lifting of the foot off of the machine, while providing a fairly full range of motion to the foot.

With love, Erika