Nowadays there are a lot of choices for online schools or MOOC's. How to choose the right one for you?
Well, I would recommend examining their business model before digging into the course content. Check if they charge per course/certificate or a monthly subscription. Compare based by estimating your total long term costs. Also consider how the content is made available to you. Will you have to keep up with the classes on tight schedules or are the schedules more flexible? Think what would work the best for you.
Personally, after reviewing several schools, I settled on Udacity and been with them for the past three years. What attracted me was that their content was freely available. I could watch their videos for free at my own pace. I did not pay anything until I was ready to take the plunge and enroll in their nanodegree program.
Back when I enrolled in the Data Analyst Nanodegree they charged a monthly subscription fee. So, the final cost depended on how quickly I could submit the final projects for each module of the degree. I took advantage of this model and studied all the courses before enrolling. That way when I enrolled, I paid only for a few months during which I was mainly working on the projects and did not have to study a lot of new material.
Understandably, a little bit later Udacity modified this model to generate more review. Today they charge a flat fee for each fixed length term of their nanodegrees. There are now recommended deadlines for the completion of each module but it's still pretty flexible because there are no live classes or weekly assignments. As long as you turn in the projects more or less on time and do not fall behind too much, you will graduate successfully.
You can still study in advance of enrolling but you will not save any money by graduating early because you have to pay a flat fee. This is how my experience was for my second certificate - Machine Learning Engineer Nanodegree
I recommend you to review the content of the modules closely. Udacity has expanded their roster of nanodegrees quite a bit. Inevitably, they nerfed some of the programs and moved some of their content to other programs. Also, there is overlap between the programs, so choose wisely to save money and time (which is also money anyway).
As of today, Udacity has almost 200 free courses for you to study at your own pace. I have completed 30+ of them so far, and I wish I had more time to complete many more. Definitely take advantage of this free content before you enroll in any Nanodegree program.
Good Luck and Have Fun!