An agent can not guarantee you a visa, nor do their applications get any special treatment from the IRCC.
It really depends what you feel most comfortable with. An agent will be able to assist you in what kind of documents to collect but at the same time you can find that information online for free. If you are competent and have the time to research and figure out what you need, do it yourself. If you just want to be told what to do, hire an agent.