While OpenSSL is among the building blocks of the Internet, I find NSS to be a higher quality library, mostly because it doesn't break API like OpenSSL does, but also because it's a more complete solution that comes with an OS abstraction layer (thanks to NSPR) a certificate storage component, and lots of other goodies. Not to mention the elephant in the room that is OpenSSL's abysmal track record, but that can change very fast for NSS as well so I don't think it's the biggest factor when comparing the two. At the end of the day, both are well-supported libraries.

The only area where OpenSSL wins hands down vs NSS is documentation, and by extension of its popularity, the easiness of googling a problem and finding its solution. As a comparison, I don't even know what the official NSS homepage is. But IME the source code of NSS is quite readable, so consider having another look at NSS if you need to deal with cryptography in your code.

Not a cryptographer, just a happy user, so YMMV.