I have been using steempress plugin for more than 1 month now. I think I will be the right person to answer this question. When I initially thought about using SteemPress, I also had the same question in mind. But the decision can be made based on your necessity.
For people who are planning to monetize your blog using other earning options like Google Adsense and Affiliate programs, I would suggest getting a professional website and domain instead of going for a free option.
If you consider SEO, contents from a registered domain will have a better SEO compared to that of free domains or subdomains. If your intention is to just use SteemPress for the sake of getting an upvote from them for using it, then I would say the free plan is more than enough for you.
I also think there are no free plan options available now. All the vendors who were providing a free plan are now charging a minimal fee for creating your WordPress blog.
If you are a proper blogger and your steemit option is just secondary, I would suggest you stick to your website and the necessary branding. In that case you can just consider steemit as an additional earning. That's all. Your primary focus should be on your blog/website.