Hi,
I’m looking into buying and building a Prusa i3 but can’t find any information on whether I should go for the 3mm iron frame or the 6mm plexiglas one. They are the same price. Enlighten me please. 
I have built a couple of i3s, non-box frames. I recall wondering this as well. What I have found is it doesn’t matter if you tap all holes. The frame can be an inch thick if you want. Thin metal will have resonance. This is something I did not think about till it was too late on my first design.
I would say the thicker, the better.
@Brandon_Satterfield is the resonance bad? For structural integrity, I would think the 3mm Steel would be better than plexiglass, or aluminum. I am probably going to build an i3 variant with 20x20 tslot aluminum.
The extruded Al would make a great frame. The one I built that had resonance issues was the classic plate design. I used Al that I cut out on a waterjet. I made it too thin and at high y frequencies the upper frame would shake. Went with 3/8" it went away. Damping/rigidity was much increased.