If I have a choice for downloading a file in .MID, .MGU, .SGU, etc. is there a prefered choice or are they all the same as far as format is concerned? What are the differences?

I created a piece and BiaB initially saved it as a .SGU file. Later on I opened that same file, worked on it for a while and this time BiaB saved it in .MGU format. What gives???

Thanks in advance