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I think it depends. If they have a budget and time they would prefer one on one course but if not they prefer online courses. For me, I prefer online courses I always watch Engvid it's a free tutorial English language. I can check out their vids in my free time so it's very flexible and no payment as well. They have tons of subscribers around the world. They have YouTube channel and a website. It's also good to have one on one course because you can practice what you've learned but it's also nice learning new tutorials in online it's a never ended learnings. You can check out Engvid.com to see what I mean.
I'm an online English teacher for Chinese student. I've been doing this for 4 years now. Improvement really depends on how eager the student is. may it be online or in school. I have students who have been taking lesson for months or years. Yet their improvement are small. Some students are focus and serious about learning so a few month they get better. Practicing the language is important as well. Like all the time. Because even though you take classes and yet you don't practice, definitely the improvements will take ages.
In as much as I've not tried learning any language on the internet, I think that it is all about the motivation that comes with learning a new language. One thing that I always do when it comes to taking up new adventures in life is reading up reviews of what those that have tried that got to say about it.

Some have said that it is possible to learn new languages on the internet with ease and that all one need is to focus and everything will fall into place at the end.