Doesn't really matter. I'm using 10.10 because I've heard bad things about the new desktop manager in 11.x
If you're going with Ubuntu I would recommend either 10.04LTS (Long Term Support) or 10.10 if needed. Version 10.10 is recommended if you need HDMI output support for your video card (although, IMHO, it should be considered beta support). Version 11.04 introduced the Unity desktop which is planned to replace Gnome either in the next version or the one after that. You can still get the 'classic' desktop from an option in the login manager. IMHO, Unity is a step in a very wrong direction. For that, and a few other reasons, I've stopped using Ubuntu and gone to Debian (squeeze).
Unity: If I wanted to run Vista then I would install and run Vista. But I don't want to run Vista so stop trying to be like Vista. kthxbye