Which format do you think is better to use?

I personally like the DSO format because the XSLT is much easier to write, i can just match on the element names. With XML result every field is in a

tag and the field names are in a header. This means I have to use all kinds of XSL tricks to make sure that my stylesheet works if the order of the fields is different in the export.

The problem is that it appears that FileMaker has deprecated the DSO result format. Why? Should I really not use it?

For export and transform, the DSO format is not a problem. Of cours, FileMaker has deprecated the DSO result format, but this format still exist... so you can use it : it is indeed more easier to use.

For import xml file : you must transform the xml document to the "XML result format".