It's hard to tell the difference between this model and the 6.6 model. Both have similar specs and features. In the end we chose this model because of the 5 year parts warranty, more of an insurance policy that we just considered an extended warranty fee for the difference. The treadmill is built very well, strong, sturdy, some of the plastic pieces are well, plastic, but appear to have a long durability factor. Fold up and down is smooth and really my favorite feature.