I have used both. I started with study because it is easier, mobile, and you can retake your quizzes until you get it right and the final test is recorded so you can take it on your schedule when you are ready. Its great.
I tried straighterline for my American Gov. class because I love history and wanted to really read about it and that's what you do with straighterline is read. The book is avail on mobile versions and you can buy the book. The final is scheduled and you are conferencing with a real person watching you while you take test. I actually liked that better than study.com due to clarity of expectations and approval of environment right away. They used to let you use book or notes but as of a year ago that changed. So, check their site for the allowed resources on test and that may help you decide too.
So, study is much easier but for certain courses I will be using straighterline.