I'd advise Linux Mint for beginners unless you want to really get into Linux (Arch, LFS, FreeBSD).
there are plenty of important differences between the two:
1. linux is just the kernel, all apps and userspace are created seperately and then put together by the Distro's, *BSD's are complete OS's, that have very little to do with each other, OpenBSD and FreeBSD are compatible with each other, and share an ancestrial code base, but they are completely two different OS's with different codes and kernels, if anything *BSD is like Unix now days, a set of standards.
2. communities, the linux communities and distro's are more concerned with marketshare and taking it to "the man" and stressing moving away from "propetiery" software, *BSD is more than happy with what they got in marketshare, don't really care if it goes up higher than it is, and has had a better relationship with "propetiery" software makers than Linux has had.
3. licenses, linux's is licensed under the GPL which is more restrictive, has more favor to the end user than the author of the program, *BSD's license is more "permissive",it allows the author a bit more control, they have a choice in the matter if they release the source code or not, or release modified source code of another project, an example of this is MacOSX, it took some things from Freebsd, modified them, packaged it up and called it OSX, but didn't give those "changes" in their full back to the community, which isn't a bad thing, some just view it as being wrong.
even tho they have slight differences, there is a lot of similarities between the two that can't be overlooked, and do make a lot think they are one in the same, they general use the same application source, typically pulling from GNU, and using the same GUI's that are available, like xwindows, gnome, kde, etc...