Starting with installation there was no real trouble. Ubuntu had it's own partition manager and it automatically created it's own partition. For the Windows 7 installation I had to create a new partition. Not hard to do, but still an extra step. Windows 7 RC had some problems with corrupted files (which weren't really corrupted at all) but this might fix itself for the RTM version whenever it is released. Both OSs installed in 15-20 minutes.
Windows 7: During the install I was prompted for the WiFi password. When I supplied it, Windows automatically installed drivers for my videocard and for my webbcam. Impressive considering every driver installation is manual on XP and Vista. It didn't install drivers for my touchpad, or for the volume buttons, on the other hand. Again not hard to do manually, but would be great if it was automatic. Programs installed with Windows 7 are the usual system tools, browser (IE8) and media player along with some games Vista users will recognize. The defragmenter has improved a lot and you can now see percentages instead of the irritating rotating blue circle. The taskbar and the new desktop options (including comparison view between two windows [UPDATE1: This is available in Vista too, right-click the taskbar and choose Show windows side by side]) are great and might save a lot of time. What's irritating (and really so) is that Microsoft has stuck with User Account Control (UAC) which asks you every time if you want to continue with the started action [UPDATE2: You can turn it off easily, or change the way it notifies you, by typing UAC in the start search field]. The security center also dislikes my choice of Antivirus program (COMODO) and flags it as a security issue in the taskbar. This just makes me want to disable the security center all together (which I did on my Vista sytem). If Microsoft cares about security it should be something that just works in the background instead of annoying users. On the upside Windows 7 RC only takes around 500 Mb in RAM compared to Vista's 800 Mb.
Ubuntu 9.04: Booting for the first time it told me there were proprietary drivers for my videocard. After a restart I could use the good looking and sometimes irritating Compiz Fusion which handles all the effects in Ubuntu. As it turned out Ubuntu had installed drivers for all my hardware and I was ready to rock (some issues with the webcam, though). There are so many programs installed, ranging from office tools to games, and you really wonder if you're ever going to need them. But it's there and it works. What doesn't work is power options. I have run Ubuntu since 7.04 and it has never been able to dim my display natively (nVidia users might wnat to check out smartdimmer or nvclock) and CPU scaling (using lower processor frequency to save power) has more or less to be done manually to save power. I have a quite batteryhungry setup but all I can muster with Ubuntu i 45 minutes of batterytime compared to 1 hour and 20 minutes with Vista (Windows 7 hasn't been tested yet).
And when you get to media and flash Ubuntu feels hopeless. The theory behind Ubuntu is that it's supposed to be completely free. No codecs for third party movie formats, the codecs included are for ogg movie, an open source format. It's fairly easy to install working codecs but I had some real trouble installing a flash player. It took me actually 5 attempts to install it. Not cool, and tiresome (youtube addicts might actually kill themselves).
So which is the better? The winner has to be Ubuntu 9.04; it's fast, easy to use and just works (sort of). But the OS for my system has to be Windows 7 RC. It feels just like Vista (which isn't as bad as people might think... I've had no problems at all with Vista) but it's a step up in usability. It's faster than my Vista system (but I know that in a year they'll both be equally slow) and it looks marginally better. The reason I will use Windows is actually because it's mainstream. You should note that I hate mainstream but when I need to download that specific program I just know it will be available for Windows, maybe Apple's OS X and if I'm really lucky for Linux. Skype installed easily on Ubuntu but I've still haven't got it to work. If you stick to the applications installed with Ubuntu it excells over any Windows version, but if you really need those specific programs you will have to stick to Windows. Check out WINE for Linux if you want to try and run Windows programs under Linux.
The final question then: will I spend money to upgrade to Windows 7? Unless it's close to free for existing Vista users I don't really see the point. I like the new taskbar but not enough to pay for it. Since my Vista sytem has worked without any problems an upgrade to a similar OS seems ridicoulus. But an upgrade from XP suddenly makes sense.
UPDATE 3: Now I've used Windows 7 moderately. I've installed OpenOffice.org, Spotify, Chrome, Live Essentials (with A-patch) and K-Lite Standard Codecs 4.9. Everything works but some things work better than others. Chrome takes up very much CPU time, sometimes up to 100% which causes the system to freeze. A reinstall didn't help. This leads me to Internet Explorer 8 which is really snappy and quick in Windows 7. And that is the first time Internet Explorer and quick has been used in the same sentence. I can also inform that Windows Media Player 12 is no big difference from WMP11, looks slightly better (especially in full screen). However I had real problems playing HD content on my harddrive and on youtube. Maximum 11 frames per second which isn't watchable. I tried VLC but it had the same problem and couldn't play the audio (same problem in Vista and Ubuntu 9.04). Not everything is perfect then. On the bright side Windows 7 is really quicker and uses between 500 and 600 Mb of RAM.
UPDATE4: After writing the above I remembered what I really wanted to say: within 2 or 3 seconds after login you're connected to the internet! That's really quick compared to Vista. Sleep and waking up speeds has also improved. This is good news if you're mobile and want quick access to the internet after login. (The internet connection thing is supposed to be fixed with Vista SP2, which I haven't tested but should be released any week now.