I am going through the same question for my build attempt. Some example calcuations are in post # 50 of the build thread:
In general, extrusions are looking stiffer than tube sections for a given size and from the same materials. Of course, if you switch from Al to steel, then you can multiply the calculated numbers x 3 for comparison purposes.
The cost of using extrusions is not only the frame material cost, but also the connectors. A number of DIY builders are spending nearly as much on connectors as they are on the T slot frame material. I think the key to using T slot economically is to use a type that has the screw holes already extruded into the cross section, and then use screws through the slots to connect it whenever possible rather than the specialized joint connectors.
I found a good welding shop locally that is willing to work with me on a steel tube frame but I have not found a great way to build with Al tube, especially if I need butt joints. Of course, if I use joints that hang off the sides, then you can screw and epoxy, but I am trying to avoid this joint type due to space constraints.
Welding 4 x 4 x 1/4 in steel tube is roughly the same price as the steel, and going smaller does not really save any money on the welding cost, at least so far.
As far as "flat", none of the materials I have looked at so far are flat or straight enough for a wood router "rail support" without some kind of improvement process. Some people shim, and other use self leveling epoxy, but plan on doing something.