Subject to vs Owner Financing

[Omari Brown]

I believe I have a pretty good understanding of both but I could use a scenario or two in which you would know which method to use to acquire the property. From what I understand, if the owner has the home completely paid off then you would try and get owner financing because if they still hold a mortgage then there will be a due on sale clause or something when they sell the home? But with a sub2 I would "assume" the mortgage and pay the mortgage on the sellers behalf? Also, if I'm "assuming" the mortgage with the same bank, would I need to be able to qualify for a mortgage for that bank or could I use another source of financing? 

I need to get these books off of my Amazon wish list and into my brain faster.