It doesn’t really matter which distribution you use. Whether its ubuntu based or debian based or something else, the underlying technology is still mostly the same, and you can always configure it to suit your needs. If you want to take most out of your linux system especially on music production, you’ll need to do at least some tweaking anyway.
Generally distributions with latest set of open source system components (kernel, x.org etc.) should have the best performance in most cases and support the latest hardware, although there’s a bigger chance of discovering some rare bugs. You should always pick up the latest version of any distribution for the same reason.
Biggest difference between desktop environments or window managers is whether they are opengl based (gnome3, kde4, unity, compiz) or use xorg’s xrender (xfce4, most standalone window managers like openbox), some like e17 can do both. Opengl based window managers shouldn’t be any slower if you have proper drivers for your card. In theory they could perform even better than others since they should be offloading the rendering work from cpu to gpu.
Closed source nvidia driver and open source Intel driver should work great. For ati I only have experience with open source driver which works ok at least for slightly older cards, although you might want to enable glamor acceleration if possible and if its not enabled by default. I’m under the impression that closed source ati drivers could be quite troublesome, and not much better than the open source ones performance-wise. Unfortunately neither ati drivers are in par with windows drivers yet.
Another thing that has an effect is basically how bloated the base system and desktop environment are. For example if there are printing services, file indexing etc. that are running on the background even though if you don’t really need them. You can always go ahead and uninstall/disable stuff you don’t need, or if you want to get really ocd about bloatware, you should look into arch linux or gentoo linux for example, that let you install and configure your system components from the scratch.
Desktop wise, the lightest solution would be running standalone renoise session without any wm’s or other stuff. Next best thing would be building your own desktop environment by picking some lightweight window manager, panel and whatever you want. There are tons of alternatives although some of them could have been unmaintained for years and not working too well with some more recent standards.
Then there are the lightweight desktop environments, like xfce, lxde, e17 and probably others. The full blown desktop environments like gnome, kde, mate/cinnamon and unity, are slightly more resource hungry, but if you can disable all the stuff you don’t need they aren’t much slower than others on decent hardware. I’d recommend simply trying them out, until you find the one you’re most comfortable with. I think opensuse has at least gnome, kde and e17 available to try out, fully integrated with the base system.
From the music production perspective the only tweak you really need is enabling the use of realtime threads for your user. You could also installing realtime or ck-patched kernel or similar, if they are available for your distribution, they should help latency related performance. There are some other tweaks you could do like setting irq latencies and whatnot, but I’d recommend only looking into those if you have issues.
imho, the power of linux comes from freedom of choice, and while some distributions make this freedom more easily accessible, no matter which system you’re running its still there. When you’re coming from OS X or Windows world, this may be overwhelming at first, but if you give yourself some time to try things out, eventually you’ll find something that works for you.
sorry, this got way too long. I hope I made my point clear and this was helpful for someone 