Radio Shack has them at a fair price, others do too as you have found. The Radio Shack model, which is likely similiar to the 2 amp NAPA version, is not what one would call a long term investment. In other words, they usually do not last too long. The Cat convertor is top notch. I've never had a problem with them. They are for the serious user however, very expensive tho not so bad as when they originally became available back in the late seventies.
There is a good reason why not to hook to one battery only, doing so puts a constant load on that one battery. The alternator, being stupid, thinks both batteries are needing a charge, and proceeds to do just that. This tends to cook your batteries and shorten their life. There is a Cat Service Bulletin about that from years ago available. Ask your Cat PSR guy to look it up for you. AC Delco had a brochure about it as well.