Suppose you're sold on the idea of XHTML instead of HTML. Then, the next thing you have to decide is whether to go for XHTML 1.0 Strict or XHTML 1.0 Transitional.
The names speak for themselves. Strict is strict/rigid, Transitional is transitional/forgiving. The benefits of Strict over Transitional are similar to those of XHTML over HTML in principle. The question is, is Transitional not good enough? Do you have to go so far as to use Strict?