I don't know the specifics but the J-1 is offered as I understand it on the basis of HAI's part 141 programme (FAA integrated professional training programme).
If you take the M-1 option to do part of the training, you then could only qualify for a J-1 if you started from scratch again, which of course would be totally pointless since you'd be covering a number of qualifications you already have.
It's a few months ago now, but I think there may have been some time restriction between VISA applications too (possibly several years).
Either way, it's a good idea to talk with the school direct since they'll have dealt with a few things. Also given the jumpiness in the US right now regarding foreigners, I would be careful not to put yourself on the wrong side of the INS. The US embassy website also has a fair bit of data.
Have fun.
r.