I bought a stress eraser last year and liked it a lot, so I gave it to my son who was stressed out in law school. Instead of replacing it I decided to give EM Wave a try. I think many people have trouble choosing between these two similar items. Maybe I can help. First, I can tell you that both are useful. You probably won't go wrong with either. However, they are quite different and you may prefer one.
Stress Eraser- strengths
Stress Eraser provides true biofeedback in the form of a continuous graph that you can influence by changing your breathing or even thinking. If you follow their instructions you can learn to control the graph and doing so you learn to relax. The unit is solid. It is scientifically valid and it seems like it due to the accurate feedback.
Stress Eraser- weaknesses
To use the stress eraser you must look at the graph that forms on the unit. I can't see it without my glasses. You must also have your finger in the correct position on the unit. That makes it hard to position the unit so that you can see the graph while lying down. Thus, if you want to use it to help go to sleep it is difficult. Another thing which may be a weakness or a strenght is that it is more or less a one trick pony. No bells and whistles. But it does what it does very well.
EM Wave- strengths
EM wave is more complicated, but not hard to use once you read the instructions. The unit is light weight and has colored lights, which you may like or feel is too much like a toy. I prefer the graph of Stress Eraser if I am using the unit during the day (it is more rapidly responsive and just seems more scientific), but at night trying to go to sleep the EM Wave is better because you don't need to see it very accurately...just follow the blinking lights. Another advantage of EM Wave is that you either put your finger on the unit (like Stress Eraser) or you can attach an ear monitor. The ear unit is great because you don't need to keep your hand on the unit. To use EM Wave thre are three different modes. In the basic mode the unit has a panel of lights that indicates a breathing pattern for you to follow. This pattern is not actual feedback, just a suggested breathing rate (you don't need to spend this much money for that). In the Heart Rate mode (HR)it follows your heart activity similarly to Stress Eraser- so this is actual feedback. In either mode there is another light that changes color depending on whether or not you are in "coherence". Essentially I take it to mean that you are relaxed, though they talk about it in New Age speak, so it is unclear to me what exactly it signifies. In the third mode you only see the "coherence" light, which saves batteries and removes all of the distractions. Actually the unit is even more complicated than this, but those are the essentials.
EM Wave Weaknesses
The unit seems flimsy compared to the Stress Eraser. It is unclear exactly what the "coherence" indictor actually indicates. The coherence coach CD is hardly worth watching. The website and other information is a bit non-scientific with all the talk about "heart breathing". All of that new age stuff will work, but depending on your personality you might find it a bit much.
Final comments
OK so it sounds like I like the Stress Eraser more...but actually I find the EM Wave more useful due to my intended use of falling asleep with it. If you use the HR mode on the EM Wave it is more similar to Stress Eraser..real feedback. If you don't wear glasses or don't expect to use the unit while lying down, Stress Eraser is probably better. EM Wave is more interesting, with colored and blinking lights and multiple modes, but it doesn't have the same degree of biofeedback. Nevertheless, EM Wave does give enough biofeedback and if you use it you will learn to relax. Again, both are good...just different.