Usually one domain is dedicated to one topic.And quality is raised so that better traffic and earnings could be. But what if you have two topics. Suppose i m interested in Bluetooth and in Astronomy then what i have to do buy two domains? or used subdomain And total concern is on better biz like good PR etc.
If both of the topic are very different, buy two domains, if they have something in common, think of using either a subdomain ( for a large site) or a subdirectory (for a smaller site). If your concer is PR etc., the best way is to use one site for everything. The PR will be then split between all the themes and the whole site will be bigger and will have better ranks. If you are just starting the site, I recommend to use 2 different domains, it may help much in link development and promotion. just my 2c
I would suggest firstly use subdomain option build the contents.If you will feel you are doing well then buy a domain .
It depends entirely on what you are wanting to achieve long term. If you're looking to build a single large brandname (think about.com) then go with subdomains. If you're building single individual sites (with no commonality) then buy separate domains. Remember that it you go down the route of sub-domains then you'll find it much more difficult to sell a single site in the future.
unless you had a domain that was rele wacky and contained all your hobbies like stars.wacko .com films. wacko .com etc i would buy a domain