I tend to use XHTML due to browser compatibility. In almost all cases where I've worked with XHTML, the content has displayed in every browser the same and the times it didn't was due to certain browsers coughIEcough not supporting the CSS styles I was using.
When HTML 5 was released, I tried it and liked it however not all browsers supported everything it had to offer and some didn't handle it as expected. Over time though, this has changed. I do find that now, it is a lot easier to make a site compatible with a large range of browsers in both HTML 5 and XHTML.
The reason I ask this is because recently, while working on a website using XHTML, I came across the need to use an iframe which isn't supported in XHTML but is in HTML 5 and that got me thinking as to why I haven't changed to using HTML 5 now that it's more cross-browser compatible than it was when it was released.