Personally I think a structural skeleton with the intention of adding strength is wasted money as it could have negative effects long term and interfere with resonances which is the main reason for epoxy granite. If the skeleton is to hold mounting plates etc and was a minimal wireframe affair then that's slightly different.

Where I think most go wrong with Epoxy builds is not putting enough effort into the molds, I would Save the money for laser cut steel and put it into a better quality and strong mold which would hold mounting fixtures, etc and allow for good vibrating. This is how the Big boys do it.!

Regards the Moving gantry or fixed then from the 2 you show then I'd say the Moving is the better stronger option. The fixed Gantry model has too much overhang on the table and when cutting at the outer edges you'll get vibrations.

If you support the table better along its full width then the fixed Gantry is by far the stronger choice but not how you have it drawn.!