The subject line has it right. Without knowing what you plan to use the system for and in what kind of environment it will be in there is absolutely no way to advise you. Indeed the article itself reeks of flamebait.
That said, I can say that I am extremely happy with FreeBSD, but I haven't played with OpenSolaris so I can't make any claims that FreeBSD is better. One of the reasons that I moved to FreeBSD (from Linux) was the more coherent administration. Every Linux distribution that I tried always tacked on a set of system administration/configuration tools that could do 90% of what I needed, but not the rest. But if I tried to do things by editing configuration files manually, sometimes the system tools would step on what I did. With FreeBSD it's pretty much all done by hand editing configuration files (except for user management, where one should let pw(1) edit the files for you). So I find that much easier to maintain.
As mentioned, the ports system is great. I find this the best package management system I've used to date. And it is easy to add a port when needed; so if I need something that isn't in ports, I can create my own port for it (which will deal with dependencies for me) and submit it.
ZFS is now fully supported in FreeBSD8. I haven't used it. I was disappointed that ZFS was not developed for OS X because I was hoping to have a truly native common filesystem I could use both on my servers and desktop. (OS X can cope with UFS, but only in a limited way).
Another things that is nice about FreeBSD (and is presumably true about OpenSolaris as well) is that the base system and the kernel are maintained by the same team. That is, these are full operating systems instead of just a kernel in need of a distribution.
The parent provides some good argument for using OpenSolaris. I'm not disputing those, but the choice depends on your particular needs