Mandriva come in two major package one is powerpack which is not free and contain non-free software and also some proprietary packages. The other option is mandriva free which is free for use. Even mandriva free give you a good experience out of the box as it contains many packages which a newbee will look for them.
Ubuntu is not a mega-distrobution so it needs the guy to add some packages by themself.
Meanwhile i suggest you try OpenSuse 10.2 which has the most new packages and gives you a good experience out of the box.
The important things to remember are that (1) there's no "best" distribution; it depends what you'll do with it, and how you enjoy doing things; and (2) very few people have actually tried even all the distros mentioned so far in this discussion; and (3) this won't stop people from expressing opinions which may or may not be correct -- as I will now do.
Personally, there are only four distros I've used heavily, and a few I've touched lightly. I've extensively used RedHat up till RH9, Fedora Core 2-6, Mandrake 9 and 10, and of course EdWare (a friend's distro, back in the 90's ) I've also briefly used Knoppix and Ubuntu.
Of these, Fedora has been my favorite. It keeps up very well with new releases of important software, and it's relatively robust to tweaking and modification. It's a pretty good "hobbyist" distro.
Mandrake/Mandriva has been my least favorite; I found it extremely fragile. "Official" update sites would routinely be corrupted: missing files, wrong file versions, etc, and this would result in the package manager being very confused and unhappy, and it was difficult to repair things. Despite the claim that Mandrake handles media-type things out of the box, I found it to be really rather poor at dealing with sound and video. I actually paid for the "Pro" distro and a "Mandrake Club" membership (or rather, my work did), and I found the website confusing and the service poor. They still send me spam. I would avoid them like the plague.
Ubuntu is supposed to be good. A lot of people I respect say good things about it. Seemed fine to me the short time I've looked at it.
Out of the box Ubuntu installs a rather sparse system, which is a Good Thing (tm) as the user won't be overwhelmed by things he'll likely never need (no 100 text editors for example, 10 mail servers, and stuff like that). But if you want all that you can easily install them afterwards as they are available.
It's also solid, and it just works. For someone who doesn't want to spend days hacking config files to get even a basic system up and running it's a good choice.
My Ubuntu machine has now been up for over half a year, the last downtime in over a year being due to me tripping over the powercord and accidentally unplugging the thing.