Well as for Beginner who usually have Windows background Ubuntu is targeted around MS Windows Users but It Entirely Depends upon the User Which Linux Distro S/he wants and probably on individual personal flavor for particular reason.
But as far the comparison is concerned you can find more about it Here :
http://en.wikipedia.org/wiki/Linux_distribution_comparison
At present there are more than One Hundred and Seventy Distros available all over the world,for Currrent Ratings and available distros you can visit the link below :
Well as far as popularity of distros is concerned Ubuntu is one of the fastest growing user community in the world.
Mine favorite is Debian due to its software distribution policy and Strong community base.
Suggestions and criticisms are welcome.
Regards
technogeek 
