These two are by far the most convenient databases that have made my journey of research quite effective since I did not need any other complex databases like 'Web of Science' and others.
Among these two, I love ScienceDirect more. The user-friendly theme of the database and the relevancy of the searched data is off the charts.
Every database has it's own importance. I would say that if you have a full access to those non-free databases, then you will definitely stick to them, as they provide more reliable information than the free ones. On the other hand, the free databases are easy to deal with and always in handy.
Truly speaking researchers usually follow that database which is accepted/recognised by their universities or work place. I prefer PUBMED indexed journals