Recently I came across both services. To understand them better to help a customer, I did a little research, which I am happy to share with everyone, as follows.
What is Amazon AppStream 2.0?
Amazon AppStream 2.0 allows organizations to stream desktop applications from AWS to any device running a web browser.
Amazon WorkSpaces explained
Amazon WorkSpaces is a fully managed, secure Desktop-as-a-Service (DaaS) solution. WorkSpaces allows you to easily provision cloud based virtual desktops running Windows.
Differences between Amazon AppStream 2.0 and Amazon WorkSpaces
While the two AWS services are somewhat similar with both falling under the “Virtual Desktop Infrastructure” (VDI) umbrella, it’s important to remember that Amazon AppStream 2.0 is focused on hosting individual applications on AWS, while Amazon WorkSpaces creates virtual desktops that can be used to create entire working environments for you and your team.
Pricing
For similar specifications (2 CPU, 4GB RAM), running Appstream costs $87.60 ($0.12 x 730 hours) per month while Workspaces costs $47++ per month;
Different use cases
I suspect WorkSpaces is more targeted for long term usage, e.g. employees to BYOD and use WorkSpaces with their existing device;
Appstream is more for large and quick turnover of users such as product demonstrations in a physical live event.