Is the weather a problem for MDF frames? 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?
I want to add the following to the already direct and very good answer of @Dani Epstein. It does not answer your question, but hopefully may help many people who are reading the question when choosing between the two materials.

Acrylic is less stable and will probably wear off faster than a well-built MDF frame for a 3D printer! I will address the points separately.


An MDF frame has much(!) more mass than acrylic, which will reduce vibrations immediately. Almost more important, a box frame, as typical for the Prusa i3 for example, stabilizes itself intrinsically more by its 'redundant wall' design (don't beat me up over the wording here, I didn't find a better way to describe the property that results from the towers.)
Compare these two images taken from reprap wiki Prusa i3:


You will probably not find an acrylic frame with this design. Shear stress as produced by the moving carriages, as well as z-wobble from the rods should theoretically be much less.
The acrylic might not wear by 'natural causes' since it is not a biologic material as wood is, but it will wear much faster due to handling the material, maintenance and human errors. The material is very brittle. You can find lots of reports on the web where the frame cracked or broke during setup. Here is my favorite example from someone with a lot of experience (1:20-2:30): https://www.youtube.com/watch?v=wkkVk8c8XoU
Acrylic is a bad choice in terms of its mechanical properties: It is soft (I found low tensile strength as a number to compare in the given database), has lots of thermal expansion (5 times higher than stainless steel) and melts at 160 C (so beware of anything touching your hotend). Feel free to compare it to other materials here (no MDF or plywood in that database, though ;-) ): http://www.goodfellow.com/E/Polymethylmethacrylate.html


This altogether makes acrylic a very bad choice for a 3d printer frame material in my opinion. Choose MDF or plywood, you can make it look cool by painting it, too.