This article compares the differences and benefits of downloading high resolution and low resolution video.
High resolution videos are better quality, but depending on your intended use, a low resolution file might be good enough. If you are looking to share video online the low resolution videos will still look good. Try it out to see if you like it, that's the best way to determine what will work best for you.
Note: Playback via the app is always low resolution because it takes a lot of data and processing power to play back high resolution video.
Low resolution videos are considerable smaller than high resolution videos. If you are looking to conserve space on your phone, the low resolution option might be right for you.
High resolution video will take much longer to copy to your mobile device than the low resolution video since there is a lot more data to transfer.