[mighty_bander]

For very simple usage, you won't see a lot of difference, but I would go with postgres. It has a lot of features that MySql doesn't, and isn't any harder to set up and use.

Point 2: as long as you also write software to deal with it, I guess so. Postgres has a JSON type. This question makes me want to ask what you are trying to accomplish.

Point 3: Yes, probably, and for all I know there's something out there to do it for you. I'm assuming basic usage; there is undoubtedly some weird corner of xmi that makes it a Turing-complete language or something equally ill-advised. So writing a schema converter for very basic stuff is probably quite easy, but really filling in all the junk is possibly less so.

4 appears to be a repeat of 2.