In my review of Kubuntu 10.10, I closed in saying that I probably wasn't going to keep it for very long. There seemed to be a few bugs in the UI, with some of the windows tearing. But I got used to most of the small idiosyncrasies and liked some of the features. So, I ended up running it for about 3 months. I've just reinstalled Ubuntu 10.10 on my main laptop and have once again feel much more at home. Firstly, I used a stopwatch to see how long it would take to install, remembering that I was totally blown away at the speed of the Kubuntu installation. Well, this time round it took a whopping 6 minutes, 20 seconds! As I didn't have a network connection at that point, I did have to install the extra media codecs afterwards, but all in all it was amaisingly fast!
Once started up, I was also much happier with the responsiveness - launching applications just seems much faster and makes for a better experience.
So, with my Kubuntu phase behind me and my curiosity satisfied, here's how they stack up:
Firstly, both flavours are largely built on the same codebase. The major difference is the user interface components. Kubuntu, as the naming suggests, is based on the KDE interface whereas Ubuntu is based on the Gnome interface. So the difference between the two largely comes down the personal preferences regarding these interfaces.
The Kubuntu UI looks better than Ubuntu IMO. There are tons of cool widgets to add to the desktop, the notifications are pretty nice and overall it has a more polished look. This all comes at a cost though. On my machine it didn't run very smoothly as well as the previously mentioned tearing problems. I'm not sure exactly what the problem was because I haven't experienced the same problem in Ubuntu, so I'm guessing that it's got something to do with my graphics card/ QT libraries. But remember that even though I'm comparing the base installations here, It's pretty easy to install additional components to spruce up those that you're not happy with. Eg. A new main menu.
There are similar effects using KWin to those provided by Compiz such as a 3D cube desktop and wobbly windows, but I find that the Compiz effects are a bit smoother. In general I preferred the level of customisation of the effects that can be achieved with Compiz Fusion Settings Manager.
The suspend to disk feature didn't work at all for me causing it to hang, forcing a reboot. The suspend to RAM did work correctly however. The boot times seem to be much better in Ubuntu than Kubuntu, but then again there's been much written about the work done to get the boot times to under 10 seconds on an "average" machine. I've found that there seem to be intermittent problems with the suspend in Ubuntu. This seems to be a problem with the latest releases because I've never seen this before.
When it comes to the package managers, I prefer the Ubuntu Software Centre interface of Ubuntu as opposed to Kpackagekit in Kubuntu, but they both really do the same thing.
As far as applications are concerned, there's not much of a difference. For every app in Kubuntu there's a corresponding one in Ubuntu and there's nothing that really stopping you from running KDE/QT applications in Ubuntu/Gnome. (Apart from a less optimised system in terms of memory usage.) Plus, there are many that are written for both the QT/Kubuntu and GTK/Ubuntu interfaces.
Overall, there seems to be better integration of the various non-UI components in Ubuntu. And most of the new developments such as the Me menu, Gwibber social client etc. are only found in Ubuntu.
So... Which one would I recommend...? If opinions were unanimous, this question wouldn't really exist - it would be a no-brainer. It always comes down to personal preferences. But for the purpose of some baseline recommendation...
On the performance front, without having done any formal comparisons, I think that Kubuntu seems a little less fluid because of the "heavy" graphical effects. I've heard numerous comments on how Kubuntu looks closer to Windows Vista/7 than Ubuntu does. And many people use this as their basis for deciding. I disagree - It terms of the transparent components, yes. But aside from that, Ubuntu may be more intuitive from a Windows user perspective. I also think that Ubuntu is much simpler than Kubuntu, but don't let this lead you to believe that it's not as "powerful". On the contrary - Some tasks that should be "difficult" to do like setting up a mobile wireless connection using a dongle couldn't be easier.
So... It's not that straight forward decision, but try out both an decide for yourself.