
I am a Mandriva man myself (2008.1 Spring currently) but recently had the dubious pleasure of installing Ubuntu Jaunty Jackalope.
Personally I was quite disappointed with Ubuntu, given all the glowing reviews/recommendations. And the main reason is the lack of choice; the installer does not give you any chance to choose what you want to do.
Firstly the partitioner only gives you the opportunity to partition the first disk (I found this with Ubuntu server as well). Secondly it did not update the permissions on my /home directory (which I wanted to preserve intact, obviously), with the result that I had to manually change the permissions on the /home directory as it refused to log into a desktop (KDE and Gnome).
Lastly I also did not have any choice regarding which packages I wanted installed (e.g either Gnome or KDE, either OO.o or KDEOffice, et cetera). It seems as if you are stuck with whatever Canonical has deemed to be fit for you and then having to set it up the way you want it afterwards.
In mitigation ,though, it could be that this particular distro was locked down by Linux Format Magazine, as it came from their DVD (so-called Enhanced edition) bundled with the July 2009 edition.
I also miss Mandriva's System tools and Control centre. In all I found Ubuntu much more difficult to set up (an in some cases it seems as if one would have to descend into the command line in order to tune it to one's liking) than Mandriva.
I have always wondered about the anti-Linux crowd who regularly (when the perennial question of "Is Linux ready for the desktop?" question surfaces) complains about the fact that one HAS to use the command line to accomplish things in Linux, but now realise that they might have a point (at least as far as Ubuntu is concerned).
Of course, some of my complaints may also be a KDE fault (KDE 4.2) rather than Ubuntu, given that this was also my first experience of 4 and the negative comments that abound regarding KDE4. I will reserve judgement on that until I have seen what it looks and behaves like under Mandriva.
What did impress was the quick install (less than an hour - I actually scuttled off for a while and was quite surprised to find that it had finished installing). Boot time is also extremely fast: not more than 45 seconds to login (and on a fairly basic machine: 512 MB RAM, single Intel CPU (Don't have the specs with me)).
I would say that, if you want a distro that does not demand much from you in terms of choice, Ubuntu is the way to go. Just install and be done with it. However, if you like the ability to choose how your machine is set up, Mandriva definitely should be your choice (although you certainly can also just go with the defaults).
If I may stray off-topic a bit, I find the arguments about boot times rather silly (and pointless). A much more meaningful metric would be to measure how long it takes from the moment the "on" button was pressed until you can start doing something productive (like open a document or a browser or mail).
My (work) XP notebook, for instance, takes about two minutes to boot to the log-in screen, but then takes another three to four minutes after that before you can even think of doing something productive. My Mandriva machine takes about a minute to boot, plus probably a further twenty seconds to load the toolbar, whilst the Ubuntu machine is almost immediately usable (if you ignore the silly Akonadi start-up and subsequent error screens, that I still have not been able to disable or get rid of). These are just my observations, though, as the three machines are vastly different and can in no way be directly compared (worst - XP machine, then Ubuntu and best Mandriva machine).