It is very slightly more secure, because you can't inadvertently leak internal network information out via your public DNS.
However, Microsoft no longer recommend you do it, for various non-security reasons - the two most usually mentioned are that it can confuse the heck out of Bonjour, and that non-routable TLDs are not guaranteed unique. (so, if I name my domain "production.corp" and you do too, we are never going to be able to connect them.)
Basically, the tiny security benefit is not worth the potential future difficulty. As a general rule, the first thing to do before setting up an AD is to get your DNS rock solid - so keep it as simple as possible.
Have a read of http://technet.microsoft.com/en-us/library/bb727085.aspx, particularly the section on naming your Forest Root, for Microsoft's advice.