The battle of the free LMS plugins for WordPress. While the extensions available for LearnPress are much less expensive, the quality of the free version of LifterLMS is unmatched. Ultimately you are going to need to purchase an extension for either of these if you want to sell your courses.
LifterLMS is actively being developed with innovative new features being added monthly. The code quality is much better, and their support is top notch. The payment gateway will cost a little bit more, but overall the system will be much better and requires no additional plugins or extensions to work. LifterLMS is my recommendation.