Thanks again for all of you again, I have found a same price discounted ticket ( IST-FRA-MIA on LH ) and ( MIA-DCA on AA ) for the exact same price; I would consider transferring at MIA even though I would fly hundreds of extra miles. At least, I could walk from Terminal J to Terminal D in Miami. At least MIA immigration is fast for visa-holders as they have designated immigration counters and officers which is empty or less crowded most of the time.