I have been working with computers for a while, before the world had windows and brick walls for the windows to have a reason for its existence.
almost every time microsoft changes the place of things in its OS du jour, people will go through a period of “who moved my cheese” syndrome. happened on win98 to 2000, 2000 to xp, xp to vista and my guess is that it will happen from vista to windows 7.
vista has all the functionality of xp plus a few security enhancements that may be annoy you (or not), plus a few graphical enhancements that may bog your system down (or not).
on the other hand, if you are a command-line nut like me, vista does have the powershell… but the best thing in vista (for me) is the lack of IE6. part of my job is being a web designer and i can tell you that the world would be a happier place if ie6 no longer existed. vista has ie7 by default and that alone would be reason to use vista over xp.
i guess if you are a pc gamer, then vista offers you directx 10 - just like xp offered directx 9c.
but your initial question was which one we liked most. that is a highly subjective question mostly based on personal experience and preferences. i am not a believer on the “one solution fits all” philosophy, so each person’s needs will be met differently by the different choices of operating systems that are out there.
i use both vista and xp as virtual environments (under virtualbox) running on my kubuntu desktops. i have used windows, linux and mac before and - to me - the best desktop experience is offered by kde running with compiz. my “windows-only” apps i use through wine, though a virtual box or though a terminal services server. you can publish a desktop with terminal services server or publish just the application using citrix or a cheaper solution like 2x (www dot 2x dot com).