I just completed a charting update to an application. I ended up using HighChart because of backward compatibility with IE8, which is still in major use in corporations.
xCharts doesn't offer anything over highcharts that I can see. SVG Support coming to IE9 was great but it will be a few years yet before we can depend on SVG.
Market share for IE8 and older is dropping pretty fast so I don't think a few years is fair. However you can get D3 working with older IE browsers there is a discussion on it here:
xCharts doesn't offer anything over highcharts that I can see. SVG Support coming to IE9 was great but it will be a few years yet before we can depend on SVG.