There is already SaveVersion property exposed by Project object, which is supposed to determine the Microsoft Office Project version. For some reason, it is always equal to 0. I have created an issue with issue id 12040 for investigation and you will be informed as soon as it is resolved.
Just pay attention, that SaveVersion is a new Microsoft Office Project 2007 field, so in the new Aspose.Tasks release it will be equal to ‘12’ for xml and mpp files created in MS Project 2007 format and ‘0’ otherwise.