This is personal preference though. There is no strong opinion that makes one better than the other. I personally prefer keeping the main site with www instead of bare domain. That way if I have sub-domains like blog, forum, or whatnot, they all stay at the same level. That said, I would keep www.example.com but not www.blog.example.com. That means I will redirect example.com to www.example.com and www.blog.example.com to blog.example.com.
I guess recaptcha works out of the box. But I prefer custom question answer. Especially in my forum (which is not in English) I can keep the question simple and do not expect a bot to find out an answer. This has worked great in our board so far.