The /proc/sys/ and sysctl(2) methods are really just down to which syscall is used to talk to the kernel - either way it's just reading or writing a kernel-side data structure, there's no fundamental difference there.
The /proc/sys/ pseudo-filesystem interface has the advantage that it's discoverable. The sysctl(2) interface requires userspace to have knowledge of a swag of hardcoded ID numbers identifying each node in the sysctl tree.
I think perhaps we might be talking at cross-purposes here.
My point is that when you call write(2) under Linux on a file descriptor derived from opening a /proc/sys pseudo-file you are talking directly to the kernel code responsible for updating the sysctl variables in the same way as you are when you call sysctl(2) under FreeBSD.
The /proc/sys files aren't real files - they're just a way of representing the sysctl variables in the existing file namespace.
Ah, when you said sysctl(2) I thought you were talking about the linux call - which is basically dead code. Yeah I don't really have a beef with the whole "everything is a file" design, I actually quite like it, but I really like how well freebsd has done sysctl - both the subroutine (3) and the command (8).
The /proc/sys/ pseudo-filesystem interface has the advantage that it's discoverable. The sysctl(2) interface requires userspace to have knowledge of a swag of hardcoded ID numbers identifying each node in the sysctl tree.