Not sure what you mean by "I feel less stressed". If your heart rate is in the same range with all of these different methods, then you are getting the same cardio workout. Muscularly though, you are stressing different areas of the body. Running uphill uses different muscles than running downhill or on a level surface, for example. Perhaps that's why you feel more stressed with one method than with another. I am not a believer in perceived effort methods of training, by the way. With something like intervals, there are going to be days where you don't feel like pushing and you may feel like you are going hard when you aren't. I think it's great you are training by heart rate zones and especially intense training like intervals.
I train almost exclusively with intervals and occasionally do a longer, slower cardio as a working break. My opinion has always been that intervals are the best way to burn fat and the quickest way to burn fat. Long, endless cardio sessions are a waste of time to me.
I have found that intervals have enabled me to go forever in longer sessions ,such as spin for instance, and have also improved my speed in shorter sessions. You get the best of both worlds without mind-numbing sessions of an hour or more. I have never struggled with intense things like spin but my HR doesn't go anywhere near as high as it used to before I started training with intervals.
Having said that, you can do intervals without going anaerobic. As long as you are moving the heart rate around and not letting it stay in one zone too long, then you are doing intervals.
My advice: start out doing say 3 interval sessions a week in your anaerobic range if that's what you desire to try. Watch your HR and moods for signs of overtraining. If you start showing physical signs of burnout, keep doing the intervals but back off and do intervals in lower heart rate zones for a while. Experiment a little with that. You'll still reap the benefits. Oh and DO keep mixing up the types of cardio you use for these. That's great in order to keep out of a rut.
Note that I don't recommend this stuff to beginners. This is advanced and intense cardio work!!!! BE CAREFUL!!
Good luck!
Trevor
