Budget will be the next point i consider, if budget is limited I will go with S9j or S9i, that will give you about 800th or 4.5k a month based on the current figures.

If budget is flexible, i would go with Whatsminer M21s, that will be 1.15ph or 7k a month, total cost will be at almost double 40$ per th vs 20$ plus for S9s.

However, the M21s will most likely live a lot longer and you will have a lot less gears to manage, less wiring and whatnot.