There are alot of claims about different styles of Zapper, but none of them are substantially better based on any research. Just get one according to your preferances and needs. I use the $10 Zapper from http://www.zapperplans.com (as do many others here) and I would say it's not a matter of this one is better then that one at all, it's just style and price mainly. Your Zapping success will depend mostly on commitment, so get one you will be commited to using.
Dr. Clark says "Any positively offset [DC] frequency kills all bacteria, viruses and parasites simultaneously given sufficient voltage (5 to 10 volts), duration (seven minutes), and frequency (anything from 10 Hz to 500'000 Hz)."
Though many Zapper sellers claim certain frequencies are better - I personally see it mainily as a marketing tool with no real research to back it. Terminator is a good Zapper from what I've heard, so go for it if that's the one you want, but don't count on it doing any more for you then any other clark style zapper.