I've used both standard html and strict xhtml, not much difference in anything really. Just like how people claim tableless designs are faster than ones with tables, that's usually not really true either. The actual code is only a couple of Kbs, so it wont really matter much. Images and compression of them matters more in my opinion.

Googlebot picks up any code though. I've had sites with html and xhtml that have showed up on google quickly.