Definitely a longer cycle at a lower dosage. 400 mgs per week will be plenty for good results, but there will be less chances of getting side effects, and you will be able to pack on more mass over a longer period of time. With the higher dosage, even when taken for less time, you will have more chances of getting side effects. And in 6 weeks it will barely kick in, so you will not be able to progress enough in this time.