[flexbex]

Your feed speed should be constant for a given toolpath, so you want both axes to have maximum feeds faster than what ever feed rate you choose. If one axis is slower, it will either slow the whole toolpath down to that speed, or vary in speed depending on which direction it's going (which will have surface quality and tool life implications.) Having a faster rapid speed on the much longer axis makes some sense.
A lighter gantry will lighter cuts, which either means having to go slower or take multiple passes (so negating any speed improvement.) Also it will make cut quality and surface finish worse.

Ahh sorry for the misunderstanding term lighter gantry. I attached my first design idea to hopefully clarify things. In this design only the z-axis carrier would move over the long axis. But i would now replace the long beam by maybe two stacked 80x40 Aluminium beams instead of a long steel beam. Hope it makes sense now.

Ahh and I want to quote the guys I got the idea from. Revolucion Shaping co.

[skrubol]

By having the gantry shorter, of course you'll be lightening it, and at the same time stiffening it (assuming the same cross section and material.)

[flexbex]

By having the gantry shorter, of course you'll be lightening it, and at the same time stiffening it (assuming the same cross section and material.)

Ahh ok got it. Sure the shorter gantry is more rigid and lighter then a long one. But in my case with the long gantry the weight of the gantry isn't that important. As the only fast moving part is the spindle carrier on the gantry.
So my conclusion would be long gantry is good enough for easy to cut material like foam and the short gantry version is more universal as I also can cut harder material.
Does that sound right?

[skrubol]

Ahh ok got it. Sure the shorter gantry is more rigid and lighter then a long one. But in my case with the long gantry the weight of the gantry isn't that important. As the only fast moving part is the spindle carrier on the gantry.
So my conclusion would be long gantry is good enough for easy to cut material like foam and the short gantry version is more universal as I also can cut harder material.
Does that sound right?

If you're only doing 3d parallel toolpaths that's probably about right. Problem is that for most parts roughing toolpaths are not parallel and want to follow X-Y contours of the part you're cutting, which may go in either the X or Y direction (or anywhere inbetween.) The same holds for 2.5D toolpaths.
Foam you may be able to get away without even doing a roughing toolpath, so it would probably be ok for that.