I did quite a bit of comparative research before I bought mine and I decided on the SilverGen SG6. It works great. I don't recall exactly why but I wasn't really impressed with the Silverpuppy units. The RSG-3 is a constant current unit also but it does not have any agitation which I felt was important and the electrodes are tiny. Although I do think that the RSG-3 would be an excellent 2nd unit for travel because of it's portability.
I actually almost purchased the Silver Edge Micro-Particle unit but I didn't like the air-bubble agitation method or the external timer configuration. I dug around a bit more and came across the SG6.
If I remember correctly both the Silver Edge Micro-Particle unit and the SilverGen SG6 had similar claims as far as particle size but SilverGen seemed to back it up with a clearer explanation of their internal electronics and how low voltage and constant current are necessary to get a high quality silver colloid. The electrodes on the SG6 also have a larger surface area and the casing and controls look more sturdy/professional in my opinion.
Here are 2 links:
http://www.silvergen.com/colloidal_silver_generator.htm
http://www.silvergen.com/technica.htm