whats gud from google point of view ? subdomain or new domain

[gingerlyspicy]

ok i started a new site around 25 days back in which i had one section on which i want to make a separate site altogether..so my question is shud i make a subdomain or new domain name altogether.....further how do in transfer content from that site to my subdomain ?i mean shud i use 301 redirect to subdomain or new domain name.....
or secondly shud i delete the content from my main site for that category and place that content on new subdomain as i heard having same content on domain and subdomain is penalised by google....i m highly confused...

Further is www[dot]keyword.primarydomain[dot]com] is better or www[dot]keyword[dot]net will be a better choice.....plz reply experienced people