It really depends on what you want to accomplish. You have fairly small travels, so what speeds do you want to achieve? You will be limited by the critical speed of your longest screw for maximum speed. It doesn't really matter what kind of motor you have beyond that. What speed do you want?
Also, acceleration is an issue that will affect your performance, and your machine does not look light. Do you want a speed demon, or what are you happy with? Are there two screws for the bottom tier? I see holes where two screws could go....but that would put those screws out in the open? And you have put alot of work into protecting the top screw from debris, so I don't know what you have planned.......
What kind of HP will your spindle be capable of? For a machine like this, I assume it's a big one. And I assume that this is for metal work. Your cutting forces could be several hundred pounds if you have a huge spindle cutting metal, and that is not a trivial amount, as it can be for a home made wood router.
For my own build, I was a little bit surprised to see what kinds of differences there were between 5mm and 10mm pitch. It made a huge difference.
To answer your question, really, I'd need to know the specifics, such as,
What is the weight of that big piece you made, including the table, and including the largest work piece you might put on it, and including all of the bits, like the ballscrew and motor? Everything that would be moving back and forth.
Where is the torque graph for the stepper you want to use, using the appropriate power supply and microstepping? What is the motor inertia for the motor? What are the diameter (s) pitch and length of the screw that is not yet shown but will be used to move the big piece you have shown back and forth?
What kind of servo are you thinking of, or are you looking for recommendations?
What are your goals for acceleration (in G's) and top speed (in IPM)?
What kind of HP spindle are you planning on? Do you know what the cutting forces might be? Easy enough to run some hypothetical numbers through a feeds and speeds calculator to see what the forces are if you know what kind of HP is available.