should i use Migration Assistant or do a fresh install

Before doing the migration, I would ensure that my old system is clean and operating properly. Then I suggest doing the migration route. That way you will save a lot of preferences, etc. Once done you should also check to ensure that the new MAC has the latest updates installed for the OS. Sometimes they don't because they were sitting on a shelf somewhere. A migration is best accomplished when you do it with a cable such as a USB or ethernet connected to both units. Last, check Apple's instructions for clearing your old computer to get rid of your personal data. Sorry, I don't have the link handy. apple's instructions are very useful. I would look at them first.