Disclaimer: the following is all very opinionated and 8.1 is the last version of FreeBSD I actively used. Actually, it's still running without any problems on a colocated machine, but it's a bit a "don't touch it" situation, because upgrading even a single piece of software would probably cause a cataclysm of struggle. So I don't touch it (except for the occasional move of something to an Ubuntu vps), but it works like a charm. I've tried version 10 in a virtual machine because it had a new installer, but it was not the best experience ever (FreeBSD's x.0 versions have always been better to skip, the x.1s were fine though). FreeBSD really needed a new installer and they did some great work, but it did not yet feel very solid. On the other hand, I've had to restart the Ubuntu installer often enough as well and I have a love/hate relationship with both of them.
Okay, let's hit it.
FreeBSD: it's clean, it's minimal. It's very good for learning *NIX stuff because you will have to do a lot of configuration yourself. What they do is very well done and I'd say it's hard to break it. Jails are very cool (OS level virtualization), but like everything else in the FreeBSD world, it's quite some work to maintain. If it runs, it runs and will keep on running basically forever. If you want to upgrade stuff, brace, brace. The ports tree was amazing back in the day, but has been overtaken by stuff like apt-get. Installation of bash used to be like this: cd /usr/ports/shells/bash && make install clean. And then, depending on your hardware and de ports dependencies, minutes or hours of waiting on the compilation. You can upgrade your base to any version without having to fear. You fetch a specific version, compile all the things (again, that is a lot of waiting), run mergemaster to fix up your configurations, reboot and you're back. FreeBSD isn't very cutting edge on the level of hardware support, but it comes with OpenBSD packet filter which is the best firewall ever, it's very powerful yet easy to setup. Also, the zfs support is awesome. That file system is truly amazing, very flexible and serious about data integrity. It's cool until it breaks though, 'cause you'll be diving deep in obscure Solaris documentation and just praying to get your data back. But, I had zfs on an external USB drive and you just shouldn't do that. With built-in drives you'll probably be very safe with zfs. To sum it up: if you're patient, care about technology and have a lot of time, FreeBSD is truly a marvellous operating system that will never let you down. It's solid stuff and it's serious, but it is an investment.
Ubuntu: as someone who started with FreeBSD, Linux always felt a bit messy to me. We, the FreeBSD users at the time, used to make fun of Linux guys saying they have scripts for everything. And in some way I still think that's true. ;) The scripts in init.d very often have 'issues'. Ubuntu does a lot of stuff for you automatically. Installation of software couldn't be much easier and many times there's not much to configure. A 'disadvantage' is that you don't have to spend a lot of time figuring out how the software does and what all it's options are, it just works. The way configuration files are organised is pretty neat, with almost everything in it's own dir in /etc/ and foo.d dirs for adding extra options in small config files. (This is very contrary to FreeBSD, where you have one big /etc/rc.conf to configure the stuff that starts when booting and where you can define extra parameters for the software. There's /etc/ for configuring the base system and /usr/local/etc/ for placing files for software that's not in base.) When you're used to pf, iptables is just shit (but ufw helps a lot). It's such a struggle to configure.
So why did I switch from FreeBSD to Ubuntu? Reason number one is because the place where I started working had done that. I really loved FreeBSD and I didn't really had that feeling towards Linux, but using Linux in day to day life is just a lot easier. It's more common, most people use Linux so you don't have to do 'BSD specific stuff'. Also, I loved doing all the configuration stuff years back, but I know how it's done now and I mostly just want my stuff to work without spending hours on configuration and maintenance. You can do that with one FreeBSD server without any problem, but when you have to manage around 50 servers, it becomes quite a thing. Installing a new kernel on an Ubuntu server or upgrading software is super easy and you don't have to wait for hours because you don't have to compile all the stuff.
To conclude, I'd say: just give FreeBSD a try, it's never a bad idea to have a look at what others do. I just wouldn't advise rolling it out to lots of servers, unless you have a very specific reason to do so.