Shouldn't be a difference

Bear in mind though what MAXDOP is. It's telling SQL the maximum number of processors that it is allowed to use for the operation. So MAXDOP 64 doesn't tell SQL to use 64 processors, it tells SQL to use no more than 64. SQL will choose how many of those 64 that it's allowed to use to run the query over, so it will use anything between 1 and 64.