As I understand it the main difference is over "trading". In the US system if an aircraft is over on one of the measurements and under on an other there can be a trade off. In the euro rules the aircraft must pass all the tests separately.
At the moment the big issues is what will happen in Stage 4. If the final compromise is
to weak then we will end up with the airports both in US (ie Orange county) and
Europe (ie Frankfurt) setting there own and different standards. This is when the real
fun starts.