yes, there is absolutely no point of going with 12 size rails given the size of your extrusion. 20 size will be best. 15 size may save you some $$

i went with screws as on that length price was same, but i had to go with bigger belt, at least Hdt3 15mm wide, which would mean bigger motors, bigger drives. And at the moment i am hoping to drive all with 30 euro ramps 1.4 and drives, 4 motors 10 euros each and the DEVFoam CNc which seems best for the purpose