I'm need to choose a software package for installing software. NSIS and WiX seem promising. Which one would you recommend over the other and why?
Feel free to offer something else if you think it's better than these two.
If you want to get an installer done today, with the minimum amount of overhead, use NSIS. Simple scripting language, good documentation, fast.
If you want to build MSI files, integrate with the Windows Installer transactional system, and have plenty of time to devote to learning the declarative model used by Windows Installer, then check out WiX.