I plan to use LT8609 for automotive application.
What is the better package between DFN and MSOP for this usage?
I mean about robustness.
Usually, is the smaller the better?
Both DFN and MSOP packages are qualified for automotive applications. Please see page 3 of datasheet for the automotive qualified part numbers. If I refer to web pricing based on 1K pieces, obvious difference is cost. So your choice will now be based on cost and size requirement.