I don't have a sport science degree. Your heart rate will go up whenever you exercise. I wasn't meaning that your heart rate would literally remain the same, I phrased it poorly. It's a question of what intensity you exercise at. I agree, most people don't use steppers or eliptical trainers properly, and so most people will raise their heart rate much more when running solely because of that. But even if using the machines properly, I would say that, you're much more likley to improve your CV performance by spending 20 minutes on a treadmill than you are by spending 20 minutes on a stepper. Runners World said that the latest study showed that eliptical trainers burnt the same number of calories as on a treadmill. If true then fine, but in the end, the movement is not identical and running on a treadmill is going to be a much more transferable activity. Having said that, walking in the hills will involve a much more diverse range of movements and range of motion than road running, so cross training with a stepper will be useful, but it's the treadmill which will do the most to increase your CV abilities.
Clearly getting outdoors is better for CV work than doing it in a gym, and I actually find it easier to keep my HR at a steady level outside as you can adjust your speed and so on much more easily in response to any change in your heart rate.