[ChrisH]

Either approach can be made to work but there are a few things you need to understand.

Scissor Lift
These require tight joints that only move the way intended. Sideways slop in your pivot joints will make the structure completely unmanageable. The members of the lift must also be pretty stiff, they cannot deflect much under load. Again deflections can make the lift unmanageable.

Another thing to consider is that the lower the scissors lift gets, the more force is required to raise it. The force required can be several times the weight of the lift and the object you are lifting combined. That is why springs connecting the joints help so much, they apply force in the direction needed.

However you only have one thing to control and it should be fairly easy to get precise positioning if you use a ball screw or something similar to activate the lift. This can be a real advantage if you are trying to acheive a precise height.

Double Jointed Arm
The double jointed arm has many fewer parts. This will make it structurally much easier to build than a scissors. But it will be much more difficult to position precisely. This is because a small variation in the angle of the arm at the pivot will result in a large displacement at the end of the arm. Now add the uncertainty of the second joint to that. You will get a fairly large uncertainty in your vertical position. You will also have a similar uncertainty in your horizontal position

You might have trouble resolving the joint angle sufficiently acurately to achieve precise position control. Then you will have to rely on your arm driver to compensate.

The higest stress on the arm will be when it is fully extended horizontally. Be sure that the arm will move upwards from this position while carrying a payload. Your operator may know that he or she should not exceed a certain limit to prevent arm failure. But I can guarantee that sooner or later they will make a mistake and the dreaded full extension will occur.