I would have to agree with Khowe in that there are really no disadvantages (or cons) to internet marketing. The only thing is that it does take a lot of time and patience to do it right.

If you want an immediate spike in sales and are looking for nothing but strictly sales leads, I would say your most effective form of internet marketing would be to start using AdWords or hook up with an advertising company that could design banner ads and landing pages for you. This is also known as "SEM" or search engine marketing.

If you are looking to become known in your industry and increase your customers and community following in the long term (and often this pays off much more in the end), I would say to make use of Facebook pages, possibly start a Twitter account, and most importantly have a well designed website that is not just attractive, but also informative. This will help your "SEO" or your search engine optimization.

Hope that helps!