When a lecture or tutorial is worth keeping, the first question is not "which converter" but "which format". The answer depends entirely on whether the picture carries information you need. Choosing well saves space, battery, and download time.
What each format gives you
MP3 stores sound only. MP4 stores video and sound together. For the same running time, an MP4 is usually many times larger than the equivalent audio (see how bitrate and file size work), so the choice has real consequences for storage and data.
When audio is enough
Talks, interviews, language practice, and narrated lessons are often perfect as audio. If the speaker carries the content and the screen rarely changes, an MP3 lets you listen while walking, driving, or doing chores.
When you need the picture
Diagrams, coding walkthroughs, chemistry demonstrations, and exercise form all depend on the visuals. Downloading these as audio would strip out the very thing you are trying to learn, so keep them as MP4.
Storage and battery
Audio plays for far longer on a single charge and takes up a fraction of the space, which matters on long trips and when you copy music to a car stereo. Video looks great but drains the battery and fills storage quickly, so keep only the clips you will actually rewatch.
A simple rule
Ask yourself: could I follow this with my eyes closed? If yes, save the MP3. If no, keep the MP4. That one question settles almost every case.
