Welcome to WebmasterWorld Guest from 188.8.131.52
Forum Moderators: mademetop
IMHO, sorta depends on your goals, and how you see the content of the "new" site relative to the old site.
If the content implies that it should really be a new site and you have the resources and time to develop it, go for it. If it pretty close in to the current site, might be worth sticking in in the subdirectory, where you can put all your admin and marketing against the one domain.
People can sometimes fail simply by spreading themselves too thin.