I'm not an expert on the M1 visa, but a number of our students come initially on thier M1, get thier private and no more than 50 hours past that, then return home, get thier J1 and come and finish, giving them alot more time on thier J1. The more time you have on a J1 the more hour building you can get and if you take a while you aren't hampered when looking for a job.
Some have said you can't come on an M1 and then a J1, but I personally know quite a number of students who have done or are doing it right now.