Treadmill is easier, with the lack of air resistance, softness under foot, and lack of variation in the surface. But there is the boredom factor.
I put the treadmill on an incline to replicate air resistance, but still do half of my running outdoors, as the hills and uneven nature of the paths and roads gives my ankles and legs more of a workout, plus the scenery is more interesting. Much prefer running through woodland than watching MTV base. Only advantage for the treadmill is knowing my pace, but one day I'll buy a Garmin Foerunner or similar device.