There are many, many sites in the world running Linux that are staying up for very long periods of time. My own personal OpenACS-backed site has been up for 145 days now. I forget exactly why I rebooted it but I'm sure I had a good reason and am absolutely positive it was voluntary and intentional. The openacs.org site's been up for 118 days. The machine running my photo site (donb.photo.net) has been up for 139 days.
These are reasonable figures for most people's websites.
The main argument for running Linux is that we know for certain that all the pieces needed for OpenACS install with absolutely zero hassle on RH 6.2 and similar vintage 2.2 kernels and the associated glibc. Most of the ACS folks who are running on x86 hardware are running on RH 6.2 because Oracle supports Linux and in particular documents and tests against RH releases. While you're using PostgreSQL rather than Oracle, I'd say your chances of getting instantaneous help from folks here are higher with the Linux platform.
Note my argument isn't based on the least on "which operating system's better". My argument is based on the fact that most of us here, for better or worse, are running Linux and therefore you're less likely to run into installation hassles if you do, too.
Also, my arguments against running FreeBSD apply equally strongly to RH 7.0 Linux.
