In my view, the established site has the advantage of being already set up. The site has been in existence so there must be some content, and that means lesser work for you. (Of course, whether you will keep the content depends on the quality as well as your goals for the site so you may still need to scrap the whole content and replace it.). Check out Archives.org for a cursory check of the history of the site and its contents.
The main benefit, however, is the aging of the domain name, the number of links the site already got, some established traffic, and possibly a steady revenue stream. Instead of waiting for a year for solid income to come in, you can have income on your day 1 after purchasing the site. Plus, the site may already be ranking well in many (or at least some) of its keywords.
The problem with the established site is that you don’t really know what the previous owners did, and whether the site got penalties from the search engines. The worst thing you can do is buy an established site, and then find out the site has been slapped with penalties with Google ad other search engines and even delisted from their databases. Or you can’t monetize the site because it has been kicked out of Google Adsense and other ad programs (once kicked out, you can’t reapply).
With a new site, you have the full control of what you do with it and you’ll be responsible for charting its growth. No penalties, nothing. BUT — most new sites land in the sandbox so it takes time for it to be visible in the search engines (sometimes as long as 6 months to 1 year). That means you’ll work overtime to get traffic to the site, even spend more in terms of PPC for lack of organic traffic.
Buying an established site vs. creating your own site has its pros and cons. But if there’s an opportunity to buy a clean, good site with a reasonable price, then it has a lot of advantages. Choose unique sites that have been around for at least a year. Avoid the dime-a-dozen copy cat junk template sites for sale that uses the same content as hundreds of similar sites for sale. Check their present customer base, liabilities if any, average sales of last six month, profit and loss account if possible and overall judge the profit making ability of that site.
And don’t ever believe the hype. If a seller is claiming the site is earning $10,000 a month or so, why is the selling offering the site only for $200? Verify the information yourself (even screenshots of their accounts such as Google Adsense revenues as those screenshots can easily be faked). Request the login information and verify the information yourself, even the traffic (check the log files or access their web traffic software).
Like any other business purchase, investigate and do due diligence carefully when buying a website.