I would recommend trying a few live cd's and discovering exactly what you want in a distro, which GUI you prefer (KDE, GNOME...). If ubuntu works for you, then by all means Ubuntu
. If you want too dig into linux a little more you could move to debian or slackware. But in the end you will be using the distro you choose, so best advice is to try them.
If you like a challenge and tweaking your system you might prefer another distro (Slackware or Debian). If you reaaaaaly want to get into linux then Gentoo of LFS would likely be the target (but not the best place to start).
The beauty of linux is basically:
You have many choices that range in difficulty of use and this is by no means a comprehensive list, just an overgeneralization based on my experiences with the distro's.
Ubuntu, Opensuse = nice User friendly for beginners or those who don't wan't to be bothered with manually configuring and installing everything, large support base, loaded repositories and is usually stable on most systems.
Debian = usually rock solid, more difficult to learn than ubuntu but also has a large support group, again loaded repositories and stable version is know for STABLE.
Slackware = Rock solid once setup, lightening fast, usually installs pretty easily but does require the use of the comand line to tweak the system. Has a large support group, and there are lots of places to get software packages but you might have to search or you can compile them yourself( compiling can be done with all previously mentioned). Has a minimal package manager that can be installed. This is what I started with, I have tried many others but use this because I'm most comfortable with it.
Gentoo, LFS = might want to get the books for these. I did install gentoo without too much of a problem when I was first switching to linux but the book is necessary especially as a newbie to tweak it and LFS might be a bit much (although the book is quite detailed). But they were also lightening fast and rock solid once up on my system.
In short, best to try out a few distro's and decide which best suits your needs. Best wishes and welcome to linux