Man on man marking
Some say that man on man marking is useful against lower quallity teams (too long to explain why...).
Mine opinion: It should be related with formation. Basic example: opponent uses 442. If you use 4141 then you should go with zonal marking, but if you use wide diamond or some assymetric formation against it, then you should use man on man.
Question is how to know which one is right? It is hard to find out, but you need to watch after opponents shooting stats. If he has a lot of shoots, then your defence isn't working properly, but main cause aslo could be formation+tactics or form of players. You can learn that from experience because game engine is quite weird.
Long passing
This is easier question and has confrimed answer.
Use long passing when opponent has 5-6 players in mildfield or when you are defending.