Mepis is based on Debian, with its wellknown (rightfully so) apt-get system. It installs binary packages.
Ports installs packages from source. Although PCBSD's pbi packages are steadily growing, it's a long way from the 12,000 or so ports. (I don't know how many apt packages there are, but I never ran into something in Debian that I had to compile from source, save for the rare times I wanted to hack the package).
Debian, for many reasons, tends to lag behind with some versions of software. Ports will often have a more current package than what you'd find in Mepis.
FreeBSD, and therefore, PCBSD also has binary packages--if one is available, you can add it with the command
pkg_add -r <packagename> in the same way you'd use apt-get. (I never did use synaptic, so not sure about that one--but if you were used to typing apt-get install <packagename> pkg_add -r works the same way.
SImplyMepis is still better as a desktop O/S--PCBSD is far newer, and more of a fresh project--by that, I mean that Warren had other successful desktop Linux distros to observe when creating Mepis, while Kris is basically starting from scratch. (That is not to denigrate Warren or his project, I often recommend Mepis to people wanting to try Linux for the first time).
PCBSD has some bugs that are being worked out--of course, so does Mepis, but to the casual user they're less apparent. Linux--err sorry, Mr. Stallman, GNU/Linux, is still ahead of the BSD's in some aspects of the desktop, especially in the multimedia department.
If the multimedia isn't that important, and you basically just do some browsing, email and the like, they will seem quite similar.
I'm not familiar with the R programming language. It seems from the link you posted that the older version works.
Other minor differences, PCBSD's default shell is csh, but that's easily changed with pkg_add -r bash. (There's a pbi as well, but when I last tried it, it didn't add the line for bash to /etc/shells.)
By multimedia, FreeBSD's firefox with flash will freeze on some sites that LInux's firefox seems to handle without problems. (Drj thinks this may be java related, but neither he nor I care enough about it to research it.) Aside from that, the various well known multimedia programs are all available.
Hope this long-winded answer is of some help. Perhaps you have an extra free partition (primary--the BSD's, unllike Linux, do require a primary partition) where you can try it.