The question you need to ask yourself is "What kind of device will work best for my situation?"
Stevens has two excellent systems that I'm aware of, Thumper being one. Thumper however, requires some basic code work and because it is not a voice type carrier, is limited (of course, combining Morse Code with it might prove interesting...)
The other system he's offering seems to have multiple applications including the revival of the old Talking Tea Kettle, a Spirit Cone or Sea Shell that literally coveys the voices of the dearly departed and of course, the normal two-way system. For the price, you can't beat it!
There are other systems out there... I know of a new "light" system soon to be released as well as a very high tech unit my associates and I have been playing with for over a year now, that is cleverly deceptive, versatile and dang spooky when used a certain way. Unlike the others, this is not your standard two-way system... nothing is "audible" and yet you can make out words... even complete sentences.
Look around, experiment, find what works best for you that's the least detectable, etc.