Ubuntu is good for newbies for one really big reason -- its insane popularity means google'ing for help with any issues is much much easier than with another Linux distro. I cannot overstate how big a benefit this is, how many potential headaches it avoids. One downside is that the initial install is somewhat bare, so you may find yourself needing to look for 'basic' apps to install from day one.

I've heard good things about Mint, even from experienced users and coworkers. Never used it myself, not for any particular reason.