I too have bad knees and I do not even try to run outside till I have run on this treadmill for at least a month or so.

The 9600 I have generally gets knocked for having a thicker softer belt, which can wear out the motor and panels faster but I haven't seen much wear on it in the 5 years I have had it but I haven't used it nearly as much in the last couple of years.
I do know of a few people that have a hard time running on a treadmill, not natural enough for them and it hurts their knees.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70