The Security+ is an exam from CompTIA and is already pretty well known and talked up in the media, IT circles and so forth. Although the SCNP (and in fact, the entire SCP program in general) has been publicly available for over a year, it's still pretty much unknown, doesn't get much media coverage, and I can't find any information on the size of the cert population, number of people taking the test and so forth.
I'm not sure I can tell which of the two is "better," but I can tell you that if I were to choose one over the other for myself, I would choose Security+. That said, it's just an entry-level security cert and won't be a stopping point for infosec certification -- rather, think of it as just the first step in a series of programs that will help you prove your infosec interest, knowledge, skills and mastery.
Good luck with your cert, whichever you decide to pursue.
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This was first published in February 2003