Clyde,
Like stated above, the two are not alike, and are really for different purposes. The older C15 model is best as a utility folder and the newer C55 is really a tactical-style folder. Of course, any knife can be used for a multitude of purposes, but some designs are better at certain tasks than others. The older C15 would make a poor defense knife for stabbing if needed, where the newer could be used for that plus utility needs. Of course, it would be nice to have both, but if it must be just one, I'd personally wait for the new one to be more available, as they're just starting to come out of the factory. I have four of the older C15's, so you can tell I love that knife! But, it does have its limitations. It's a great working utility knife, though. I'm patiently awaiting the new model...