Should I guest post, advertise, etc. on existing sites where users talk about my competitors or should I find new sites similar to my target audience?

I'm able to find find blog sites, forums, and other sites where I see users talking about my competitors. A part of me feels that trying to reach these same users will make it easier to acquire customers because they already know how my services work because of my competitors; however, a part of me feels like that I would not acquire any customers because these people are most likely already using my competitor's services. Or another route I can take is to try to find new sites that have users that match my target audience but have not heard of my competitors or the services I offer. However, in this case, there is a bigger learning curve in having to explain how my services work and then seeing if this is something they would even be interested in. At least with taking the competitor user route, these users already use similar services on a daily basis; so if I can entice them more, maybe I can acquire them. What do you feel is the best approach for acquiring new customers? Go to places where there is already discussion about my different competitors? Or try new websites that have never heard of my competitors or the services I offer? Thank you!