Generally speaking, MDF will weather OK. In areas of high humidity you might experience warpage, but you can mitigate that by sealing the surface with paint or varnish. However you will probably find that of the two materials, acrylic will be more stable over a few years.
I've acquired all the parts to build a Reprap Prusa i3 rework, the only missing part is the frame.
I'm in doubt between a MDF cut (cheaper) or acrylic (more expensive), of course a cheaper one is my preferred option until I see any disadvantage on making it of wood.
I thought about variables like heat and humidity and the possibility of expansion/contraction of the frame, is this a valid concern? Will I have more precision buying the acrylic one or is it irrelevant?