Both are very solid learning management system plugins for WordPress and are clearly the top 2 learning management systems for WordPress.
There used to be many differences between the two, but in 2018 they caught up to each other. They now both have course builders to make it easy to set up and manage large courses.
That all changed though in 2019 when LearnDash released v3 which brought a new and completely modern design for the front end of your website. This update also brought a new feature named focus mode which makes the learner experience similar to what you find in expensive SaaS apps like Teachable. It makes it so easy to set up a course that looks amazing.
Hands down, setting up a course is a lot easier with LearnDash and it will look 10x better and more professional! Keep in mind this may change in the future, but this is how it is today.
As far as pricing goes, I think the price of LearnDash is more realistic, where LifterLMS has seriously inflated prices.
Both are great options, I think for ease of use and a better user experience you should consider LearnDash, and if you are just starting out and will only be offering free courses consider LifterLMS.