Well, there really isn't a reason this would start a flame war, T::T is a more powerful library. You have plugins from all sorts of modules, like DBI, CGI, Data::FormValidator, the list goes on. Check out the docs for a more robust list.

Out of the two, I'd choose Template::Toolkit. But that's only because I've used both, and found that it provides more of what I need at this point in time. You can do a lot with HTML::Template, it's a really simple and neat little module, but it's hard some times to deal with when you're manipulating large and complex data structures. It would be a good starting point though.

Have you looked at HTML::Mason as well? It's a little more like PHP in the sense that you can embed your perl code right into your template, but you can also abstract all your mark up away into "elements" as Mason calls them. Pretty neat if you have mod_perl and some time to install it.

Hope this helps!