I ordered the Ubuntu version about 2 months ago. Couldn't be happier. I'm a Slackware user and have been using Linux since 1997...heavily. I've had no issue with Ubuntu whatsoever. I've seen no need to use another flavor of Ubuntu, either.
In fact, I'm also a Mac man. Will I install OS X? No. There's no reason why I should. I already have a Macbook, and to be honest, the Dell is fine. There are a few computers in my house that I'd like to keep pristine and non-tinkered. The more you play and tune with settings, the more chance you'll break something or get something out-of tune. I've at least 9 systems up and running in the house. The last thing I want to do is have to fix one of them (they keep me busy enough as it is).
Ubuntu has less overhead than XP does. It has less of a footprint, too. It's definitely polished, considering other distributions that Dell could've selected. There's no way in Hell you can do the level of tuning in Ubuntu on an XP machine. I'm saying that and not being biased when I say it. Its a simple fact that Linux is more open. Just like an auto mechanic, once you know what you're doing, Linux is childplay. Most of the time, you can fix something without rebooting.
For a netbook, Linux appears to be more scalable than XP. That may change when Windows 7 is released, but for now, the comparison is Linux and XP. There's not much you can do with Windows that you can't do with Linux, especially with netbooks. IMO, its more personal preference than anything else.
Longtime users of Windows usually have a bias that they can't shake...I'm not saying that's bad but it kinda makes those people not want to even be open-minded about comparing two different OSs. I've seen the reversal of this, also, but less so. I've Windows machines on my networks that I use for heavy gaming, so I know the value of Windows (ease of game installs, game use and configuration, for me). For resource-intensive tasks where I don't want to pay and arm and leg for software, I use opensource software and have no issues with installation and repair of apps...YMMV, though.