It depends. Before anything, keep in mind that neither option is better than the other. Both can produce either a successful application or a failed one.
Some can successfully tackle this process on their own because they have the tools, understanding, knowledge, motivation and confidence necessary.
For others, having professional assistance in this matter, is a must (Maybe they lack the time to do it themselves, Maybe they are not confident enough to do it alone, Maybe they have a very complex individual circumstance and need guidance on how to deal with it, etc)
For starter, ask yourself these questions: How comfortable are you with your understanding of the process, the requirements and with the profile you have? How motivated are you? You think nobody could submit a better application than yourself? How much free time you have to dedicate it to the application? The answer to those questions will shed some light about how you should tackle your application.
Best of lucks!