COMPARING PLATFORMS
After spending hours evaluating each of these services, the one I prefer the most is Udemy. Not only does it offer a vast array of topics, but once you purchase a course, you always have access to the content. Nothing irritates me more than needing to reference something that I no longer have access to because the subscription expired. Udemy's iOS client is also top-notch, and its instructors are trained guides that keep their content interesting while navigating you through new ideas.
My second-place award goes to Coursera. Like Udemy, Coursera's content is always accessible once purchased. Unlike Udemy, Coursera is oriented toward formal academic courses that can contribute toward a professional degree. Free services like edX and Khan Academy have their place, and I appreciate the ability to access their content without paywalls. But their catalogs are not as expansive or polished as paid services, and their iOS apps are essentially shells for the YouTube content they organize.
Lastly, a special mention goes out to Pluralsight. While its target audience is predominantly working IT professionals, its high-quality training videos cut through the marketing cruft that frequently spams vendor-supplied training. It's also no surprise that its iOS client is one of the best among this group, given the discerning technology professionals they are trying to impress.
Regardless of which service you choose, you'll find that they all offer options to test them out before committing your time and money. It's in their best business interest to deliver inspiring courses intended to make you smarter.