There are a couple of reasons to prefer 3DM if you have the choice.

One is that MoI only writes writes individual surfaces to an IGES file, so you have to re-join surfaces together in your other software. There is actually a mechanism in IGES to write joined surfaces but MoI doesn't use it right now because it is not very widely supported in other software.

The other thing is that 3DM files use a common library of code (the OpenNURBS library) to do all the reading and writing work, and this makes it less likely to have any variations in processing the 3DM file. So it's more likely to be a little more robust.

- Michael