I think what it really sums up to is that although the rustica may be more difficult to build (although only slightly) when you're done, you have a far more useable setup. I really like the idea of being able to hang the system from a table, making it possible to cut the foam directly without having to raise it up on blocks. The aeropic is designed to sit on the ground, and the lowest the hotwire can get to the ground is about 2 to 3 inches. This means that you would have to raise the foam, adding to the complexity of setup. Also, in using drawer slides, getting a travel of 30" or better would be tricky and very bulky. When the slides are fully extended, the weight is not distributed directly over bearing and  is cantilevered, which could introduce error in the cut.

Proposed part list for la rustica
I spent some time with McMaster trying to find the main components for my rustica. Here's my first cut: 

   

Description item # qnt each total Notes 
ball bearings (11/16X1/4") 6383K12 16 2.36 37.76 closest to 16x5mm (17.5x6.4 actual) 
steel tubing 3/4" 6' 89895k36 1 57.6 57.6 stainless steel 304 OD +-0.005" 
brass tubing 3/8" 6' 8950k58 1 11.93 11.93 alloy 330 brass OD +-0.002" 
alum right angle 1/2" 8' 88805k41 1 3.91 3.91  
al. box beam 2"x1" 6' 88935k66 2 32.65 65.3 Seems expensive. Anyone have better source? 
threaded rod M6 1mm 3.2' 94185A155 3 3.7 11.1 stainless steel for lead screws 
threaded rod M6 1mm 3.2' 98861A070 2 1.1 2.2 regular steel for support screws 
      
      
(updated 03/24/2002)   total 189.8  

  

Note that most items are in 6' lengths based on 3' (36") x-axis travel. This is a little shorter than the standard 1m (40") travel of the rustica, but I guess that's life. I chose stainless steel for the main x-axis tube as it is longer and will carry more load. For the y-axis I chose a brass alloy that was considerably less expensive, since it will carry less load (most of the load will rest on the lead screw.) The ball bearings that I chose are semi-precision and are quite reasonably priced. I think that their tolerances will easily be better than the tolerances of the tracks that they ride on. It seemed like all the prices were very reasonable until I got to the aluminum box beam. I guess there is quite a bit of metal there, but I was still hoping that it would come out to less than 1/4 of the total cost. Does anyone know of a good substitute? 

Rustica Mods
I've been playing around with a CAD package called SolidWorks. I've used ProE and AutoCad, and a few other packages but this is BY FAR the best CAD package I have worked with. It is a joy to use. Everything you see here was done after just a couple of 10 minute tutorials.

Anyway, getting to the point here, I had some ideas on how to improve the rustica design. First was to use a vertically oriented channel of 1"x2" aluminum to increase rigidity. Also, larger bearings (19mm x 5mm) were used as suggested by others, and a larger diameter (1 1/8") brass tube in place of the steel tubing. The modulus of elasticity of brass is only about 1/2 of steel's, but the extra size makes for a stiffer tube overall.