Hi Marcel, it depends on what you are going to be doing with the mesh and what software you are exporting to.

But in general if your target software supports n-gons it means that you will get a clearer looking wireframe structure using that option.

The n-gon output will generate multi-sided polygons in areas that have trim curves (basically where booleans have happened).

Here is an example, here I have taken a sphere and booleaned a hole out of it:



If you save this to a mesh format, if you use the n-gons option you will get a result that looks like this:



Here I have marked how an n-gon is formed in that spot where the hole was cut, you can see that the polygon is made up of many segments there:




Now when you open such a mesh in Cinema4D for example it looks like this:



The mesh just has a cleaner wireframe that much more closely resembles the original NURBS object rather than being chaotic.

Here's an example in Cinema4D of a more complex model meshed with n-gons:




However, not every program supports n-gons properly, so if you are exporting to a program that does not process n-gons well, then you will need to use the Quads & Triangles option usually. You would generally only want to use "triangles only" for special purposes if your target program does not even support quads but that is uncommon.

If you let me know which program you intend to export to it may help to give more specific advice.


> and a pure quads export is, I suppose, as in rhino, impossible?...

It can work in some special cases like if you have objects with uniform curvature such as a surface of revolution that does not have odd areas booleaned out inside of it.

But for a totally general solid that has trim curves in it, no you won't generally get all quads from it.

Getting all quads would require tiling areas in a different way than MoI's mesher currently works, for some recent discussion see here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=2451.50

- Michael