There are lots of YouTube MP3 Downloaders online, and I will do recommend TunesKit Audio Capture, a professional and powerful streaming audio recorder. You can download music from YouTube to MP3 with ease.
This youtube mp3 converter https://2conv.com/en51/ works great for me. I mostly need a lot of music in mp3 for editing and this site provides really good quality and it's also super fast and convenient. There are not so many ads and it works with a wide range of sources.
I never try the service you mentioned. But I think 2cov is safe to use. It supports MP3, MP4, and AVI. As for an offline product, maybe you could try Joyoshare Screen Recorder. It can record audio from YouTube in high sound quality and save as multiple formats.