I'd also go for ubuntu mostly because of the hardware support (almost everything works out of the box) and that it works easily with windows networks (I find configuration of samba tricky and ubuntu does it with one click the fist time you try to share a folder). I also happen to like the clean direction they are taking ui-wise. Plus it's based on debian so sure you'll feel at home. Finally it has more recent software than debian without having to dive into unstable.

I also had almost no idea about linux before the N900 and now I am a real fan. Because I still have some reasons to use windows, I have a partition with windows. and I have symlinked the Documents, Pictures etc folders in ubuntu on the windows partition so it's a breeze to switch. You have to edit fstab though to automount the ntfs partition and enable trash.

One other thing you should try is lili flash drive creator. Additionally to creating a bootable usb (try a e-sata drive, really fast) it sets up a virtualbox so that you can plug your flashdrive in any windows pc and run your linux (albeit a bit slower) without even restarting.