Does the amount of calories or the number of miles matter more when on the treadmill?

What REALLY matters is your heart rate. That's a measure of how hard you are REALLY working. This effort will differ for each person depending on age and how fit they are. Keep your heart rate in the low cardio range for weight loss. Then time / distance / calories burned will all somewhat track with each other (more distance, more calories).

Here is a calculator to find your target heart rate.

http://exercise.about.com/cs/fitnesstools/l/bl_THR...

Also, remember you can't undo a bad diet in the gym. 80% of weight loss is eating. Learn what a real portion looks like (Americans are bad at this because of all the supersizing). Make at least half your plate veggies.

For weight loss you'd want to look at calories but if you're trying to build fitness/stamina the miles is what would matter more.

Miles matter more, and time. Calories on a machine are only general estimates whereas miles and time are constants.

Calories. They are the actual measurement of energy spent.

Counting and running are often defeated by chewing. What you eat is important.

You will get better results from your exercise routine by cutting sugary and starchy food and beverage products from your diet. The more you CUT the more you LOSE.