Potential for what? Since your question is quite vague I'm going to assume you are talking about what's better for your website in terms of acquiring more business. AKA "SEO"

Forum posting is good if you are learning and teaching others by answering and asking questions and becoming a valuable part of a community. It can allow you to create a sort of online network. It may not bring direct traffic to your website, but in the right forums, you can learn a lot which will help you. Here for example at forum SA there are many business-related topics. So here you can learn or teach others how to run your business more effectively.

Blog Posting: This is good for covering more topics in-depth on your own website. Let's say you sell websites. You can write a post about what goes into the costs of developing websites. You can write about how people should choose a good website designer. These are questions most people who want a website may look up. So what we are doing here is targeting more specific pre-sale questions, you can do it with any niche. It also increases the overall relevancy of your website.

Both are good.

The key is always to try to add value and demonstrate a bit of expertise. Google notices it and people do too.

If you are talking about posting for backlinks. Blogs rarely get you backlinks, many forums won't allow backlinks because they draw spam and they also don't count as much to SEO as you would think.

If you are posting on forums purely for backlinks instead of trying to help people or to generate helpful discussions (learn, teach) you need or revise your SEO strategy.