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What I would do, is visit the sites of the companies that are offering these products, and then see what the differences are, in terms of price, functionality, etc. I know at least 1 company offering such services has a toll free number - call 'em up (if you are in the US) it's their dime for the call, and get all the info you can.
In addition, we have several members that work for / and or own companies that sell these types of products, so it makes discussing them a bit of a challenge.
I'm sure, if these products offer ROI, you can find their ads in the PPC's they work with :)