What advantages FreeBSD offers over Linux so that for the next installation on EC2 one would consider running it instead of Linux? (question is not rhetorical)
I don't think there's anything specific to EC2; so your question just reduces to "why FreeBSD rather than Linux?"
There's a lot of differences which are more matters of taste than anything else: FreeBSD's ports system, the rc.d startup system, the fact that the kernel and userland are developed in tandem rather than independently, the presence of a central SVN repository, et cetera. I like all of these, but I know there are Linux people who prefer the "Linux approach" for each of these.
Some things which I've heard even hardened Linuxites saying make them tempted to try FreeBSD, however: ZFS, DTrace, LLVM/clang (now building the entire FreeBSD base system), virtualized networking, Capsicum. Also new in 9.0, but not getting much attention yet, is journaled soft-updates -- essentially the "best of both worlds", using soft-updates for the 99% of operations which soft-updates can handle quickly and safely, and journaling the remaining 1% to ensure that a dirty fsck is fast.
I'd add that "jails" are a very cool, under appreciated feature of FreeBSD. It is especially useful for packaging up and delivering applications as a single unit. While it may seem that they are in competition to virtual machines, I think of them as being a complement.
