Since that discussion took place.
Now what many people forget is the SEO part of any version of Html.
IMO Html is the biggest tool one has at his fingertips to push a site up in ranking. Cms, something not mentioned here is all about the SEO friendly structure.
OK those systems can have a SEO friendly domain extension and have even an Alt tag. but there are more tags to a site and there are more names to folder, images (which can also be named before uploading) and else what can be done on html. For a good SEO you need to know how a spider sees your site. True now with panda 4 content has a bigger percentage but that is a lesser problem in html.
For any smaller site you need to find out what your regional clients have as browsers, where I live Iexplorer 8 is kind of advanced. A larger site like 20 to 100 pages in dimension php is way better than CMS.
Specially when trying to get people without much knowledge into designing their own sites CMS has quiet a few short comings, one mistake, one change of an image and the site looks so different people regret having even looked at CMS. Cms is sold as the new site develop software even so it just gives the designer a way shorter development time, kind of cheating IMO.
CMS is not the issue here the question is "Should I use HTML5 or XHTML" ?
Well have a look into your servers user stats and see what kind of browser they are using to browse through your site. That would give you a nice picture. You could also try to use a system where you have browsers like Iexporer6 just to see if the browser garbles your site out in a way no-one in his mind would allow that to happen to his site. You can mix even both styles and see how it is interpreted. A spider has a very different way seeing stuff, it crawls through our sites and decides what is good content and what is not. Those critters read tags more than Html5 allows and like tags even more than flashy images.
Lets have a look at your site surely by now it has been finished how have you decided?