I have been using Linux for many years now on second computers and on VM's, but about a year ago I switched to linux on my primary Laptop and I have been very happy with it. I have used a few distros (Slackware, Gentoo, Suse, Ubuntu, Kubuntu), and was happy with Ubuntu a few years ago. Someone I work with got me to switch to OpenSUSE about two years ago and it was very good--easy to administer and very well put together/documented). But after upgrading last year I was having trouble getting my aircard to work (which I really needed for work). I decided to try setting it up on Ubuntu to see if maybe that gave me some clues as to what the problem was, so I ran the live cd, plugged in the aircard, and before I could look up the instructions for setting it up, a network manager popup asked me who my carrier was and then connected me to the internet. I switched back to Ubuntu, and have been very happy for the last year, including doing development for my N900.

Ubuntu is good because it works with alot of hardware (though with a desktop that is less of a problem) and because there is ALOT of info on the internet to help troubleshoot problems when you have them. Also, many (but not all things) that are made available for general use on Linux will have Ubuntu installers. You can generally install just about anything on just about any distro (that's really part of what open source is all about), but if you are just learning you might have an easier time on Ubuntu when you don't want to spend a day learning all the ins and outs.

I also agree that the common Debian base will help somewhat with understanding how things work on your N900, but seeing how things work differently between a Yum/RPM based system and an Apt/DEB based system is also very helpful in understanding things, so you really can't go wrong either way. And you can always wipe and switch (or even dual boot) whenever you feel like it.

One more thing...virtual machines make alot of the "compatibility" issues obsolete. I do my scratchbox and qt programming work in an Ubuntu VM on my Ubuntu laptop, just to keep things separated and simple (and because I can run a 32 bit system as a virtual system on top of my 64 bit install). You can always put an Ubuntu VM on your OpenSuse desktop and use that (locally or remotely) when you want to do anything that is easier on a debian system. I like Virtualbox for that kind of thing (I prefer VMWare Server when I want to run unattended servers as VMs, but that is another discussion).

The choice is daunting because you are experiencing a new feeling: FREEDOM. Just pick one (I say Ubuntu - 64 bit if your hardware supports it), try it (just install with the defaults), and if you don't like it pick another or reinstall with different settings. You will learn more each time you install, and you really can't go wrong. But whichever you try, welcome to Linux!