I work for Wild Divine and this popped up in my google alerts, so I thought I'd provide what information I can. I have used all of the devices mentioned so far, except the Emotiv.
Generally, they are all going to help with relaxation when used properly. One thing I'd note up front is that the StressEraser is the only one that is considered to be a biofeedback device.
The StressEraser and EmWave are both good portable units. The drawback to their portability is that they are going to be a bit less interesting to use. There's only so much that you can do with such small devices. They are essentially going to help train you to breathe more smoothly and regularly. The StressEraser plots your heart rate on that display on the device, and also has a tone that is used to help train that. The EmWave uses those lights to show whether your are in coherence.
Wild Divine's products are going to be somewhat less portable because they have to be used with a computer. However, using the computer offers vastly more flexibility in what can be done with the iom hardware - which does heart and skin response. Our products teach breathing and meditation techniques to help with relaxation. In Relaxing Rhythms, which is the program of ours that I recommend for most people, there are guided practices and guided meditations that are intended to teach various relaxation techniques. Then the hardware is used to drive mini-games that reinforce the lessons being taught in the program. Using the tree example that bcingu mentioned, the user breathes with the expansion and contraction of a tree displayed on the screen. As they do, it builds a bridge until the game is complete. Or if you don't breathe in the proscribed pattern, it doesn't.
Heart Math also has a computer based product, emWave Desktop - http://store.heartmath.org/store/learning-programs. The major difference between Heart Math and Wild Divine is that Heart Math tends to focus more on numbers while Wild Divine focuses on creating an experience. So if you're looking for a program that can be used with a PC, Heart Math is the choice if you want numbers and graphs, and Wild Divine tends to be a better choice if you are less concerned with those, and are more interested in a graphically rich experience.
I have not had an opportunity to try the Emotiv system, but I'd like to. $299 for a full EEG system is crazy. Just a few years ago something similar would have cost thousands. if your wife is going to be using it in public, find out whether she is willing to put something on her head. It would attract quite a bit more attention than putting sensors on your fingers or ear lobe.
bcingu: Where did you temp at Wild Divine?