My keyword is tsjingtsjang I've old domain called www.mywebsite.com (PR3) I'll create sub-domain tsjingtsjang.mywebsite.com I bought new domain www.tsjingtsjang.info What will rank better?
If is relevant to old domain that old subdomain is better both for subdomain and you main site other wise a new one and on page rank and traffic issue i think new domain will take some more time than old one but in long run its doesn't make much difference as you will also link new domain to your old domain as you will link a subdomain.
tsjingtsjang.myPR3website.com will rank better than your new domain. the other one is already established and has more IBLs compare to the new one.