Linux is a kernel, FreeBSD is an Operating System. So you really can't compare them like you want. The major distributions of Linux each have various levels of compatibility and differences... but I'll try to hit the things you named at least:
- There are a couple different package managers (dpkg and rpm probably being the most popular) for Linux (and other OSes). FreeBSD is just switching from pkg_install to pkgng. From an administrative prospective you generally only need to know that there is package management in all of them.
- Most use the same user management, but there's no guarantee. Most keep users and passwords in a file in the
/etc folder, though FreeBSD uses a database in that folder, so again, generally the same idea, but not exactly.
- Memory, no real difference except that Linux generally has the OOM which will kill processes when you run out of RAM. FreeBSD gracefully denies requests for new memory when RAM is full, which causes poorly written applications to trip all over themselves...
- Configuration, you should probably be using some sort of configuration management tool for either. Generally it's all a bunch of text files, like most *nix environments.
- File Systems - the
ext family for Linux, and
ufs/ffs for FreeBSD. Both can use ZFS now, though FreeBSD comes with native support that Linux will likely never have because of it's licensing choices.
- Switching usually means rebuilding whatever you've got from scratch, with very little carryover. Applications will generally be configured similarly on each, but not identically.
"Tedious, difficult, confusing" - Yes. If you're familiar with one the other will be. The BSDs tend to impose significantly more structure to everything they do, development process, file system layout, etc. Linux is just a kernel, so it's up to the distro to organize everything else (they tend to use the GNU userland, but even there some have borrowed utilities from BSD and other projects). The flexibility in the Linux process tends to allow more people to participate in the process.
One important note, the various Linux distros generally like to mess with the configurations of their packages. So you'll find all kinds of crazy configuration stuff going on that is not part of the original software. They do this to make the software easier to configure - or that's the intention if not the result. The BSDs, tend not to mess with these configurations much. When you install Apache on many popular Linux distros the configuration is spread out into various directories, with configuration utilities that you'll not find on Apache's website, and more. On FreeBSD the configuration files get installed in an application specific directory, but otherwise they're exactly as they come from Apache.
I'm a wholehearted fan of FreeBSD, but you can't apply a simple "better" tag to either. Each has strengths. Linux is much more popular right now (unless you count systems that run software derived from BSD, which includes MacOSX, iOS, and a ton of Cisco, Juniper, EMC, and others equipment) . The BSDs have a much longer history, and were more popular until the AT&T lawsuit (which was settled a long time ago with no winner). And the list goes on...