which Linux emulation?

A quick pkg search linux_base reveals three choices: CentOS 6/7, Fedora 10.

Which one should I pick? I've never used either distro, preferring Debian and Arch- based ones.

pkg search linux | grep c7
shows that there is nothing available for CentOS 7, so I guess it should be c6 or f10?

Just curious, might need it one of these days. I was trying to run a linux binary BasiliskII and discovered I could compile it natively more easily than figure out which linux emulation to pick.
I think the f10 related packages are old and were deprecated in favor of c6 ports. The CentOS 6 compat is being constantly developed :).
F10 would be Fedora 10 and as Atsuri says, is deprecated. CentOS-6.x is pretty old too, actually, though it's still maintained. Even CentOS-7. x is using an older kernel than most other Linux distributions.
For now, try c6, quite old, but many packages are builded with c6 as default (flash plugin etc). Future is c7, it's already available in ports, but IMHO not as "stable" as c6 and still no complete 64bit user space/libs,