On the surface, both look like solid options for adding an online course to your website, but only one of them are really worth consideration. Sensei is really difficult to get working with any WordPress theme, however, LearnDash will work right out of the box. Also, Sensei hasn't seen any new development since 2015, any updates have been bug fixes. Where LearnDash has had countless new features added, features that you would want in an LMS. I just can't recommend anyone use Sensei because it is not a serious product for the future where LearnDash is their main and only product, so they are laser-focused on it and have added massive improvements each year.