What are the pros and cons of choosing my own research topic?
Along the lines of earthling's answer, you do have a strong say in your topic, and you'll definitely want to be interested in the topic as you'll be putting a lot of work into it.

However, do realize that, as an undergraduate, you have far less experience in doing research than your advisor. You will save yourself a tremendous amount of work and effort by working closely with him on your project. One way to ensure that your advisor will be willing to invest that time in you is to work on a project that (1) interests him and (2) is related to his research. Even better, both. You are definitely free to choose whatever project you want, but it would be a good idea to choose something on which he can offer his expertise.