I'll take a wild guess. I know that cyclists try to maintain a high cadence in an effort to increase efficiency. Assuming that this concept would also apply to running on a treadmill, it would seem that the higher resistance level would be less efficent, and therefore, a better workout.
The second question is, I think, a little easier. Sweating is caused by a combination of factors with effort just being one of those. Clothing,
body fat, metabolic rate, air temperature, level of fitness etc. are also important.
Heart rate is a much better indicator of effort.