Hey Sparky, I am just getting started with my build as well, but I can take a shot at answering your questions.
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My opinion is that you want the X-axis to be shorter (axis that the router/spindle is mounted to).
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like you state, .120" tubing will provide more rigidity and less deflection no matter where you use it. I believe the rigidity would be greater than the additional weight. Maybe somebody else can chime in and correct me, but I don’t think it would hurt to use everywhere. That said, for this machine, it may not make much difference.
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Ryan suggested using rectilinear for the reduced print time. He stated that the parts are designed to only be loaded in the X and Y axis and so rectilinear should work fine.
4.12V vs 24V - Check out this thread Building new primo quick? - #2 by SupraGuy