How much do you want to learn, and how much do you want to challenge yourself???
Anybody that can operate windows or mac os can just as easily operate ubuntu.
If you have never touched linux before, and you want to experiment first right off the bat, then you might want to grab a "livecd" distro and boot right off of a CD rather than installing .... Something like Knoppix should work VERY well for that purpose, or something like SLAX should provide acceptable usability as well.
If you are determined to install something, and you want something more challenging than ubuntu, look no farther than Slackware. In my opinion, it is the best Linux distro if you want to really learn linux. It doesn't have nearly as many bells or whistles as Debian or Ubuntu, however, with slackware, you'll learn to love your command line. As a bonus, Slackware is the most "bsd-like" of all the major Linux distros. I run slackware 11 on my file/ventrilo/web server.
Personally ... I have the most fun playing with OpenBSD ... I've got it on 3 boxes now .... (nat/pf box that I plan on turning into a WAP and VPN bridge (to secure my wireless)), as well as 2 laptops ... (one of which is an old 450mhz with just 192mb of ram. I was planning on retiring the box and either selling it, or giving it to someone in my family who doesn't have a laptop and could use it for school ..... but It's older than dirt and I kinda like having it around just for an extra websurfing box. It runs a lot faster than many of the MUCH newwer boxes I've seen that run XP ....
also, FYI, the core of OS X, the Kernel, in VERY similar to that of OpenBSD. (In essence, they are both BSDs) ... OpenBSD seems like a bit more of a firewall/server type OS than a desktop OS by default, but it makes a nice desktop, it just takes time and work. I have X running and XFCE4 on both of the laptops. One is OpenBSD 4.0 and the other is running 4.1. I just installed 4.1 so I haven't had much time to play yet. The initial installation of OpenBSD can be a bit tricky as you need to create a slice, then make partitions within the slice, rather than just make partitions on the drive(s.) But if you're a veteran OS X user, and you can handle your own on the command line as well as the GUI, then you might just be ready to play with OpenBSD ...