Why you might purchase a single study
The per study option suits people who are certain they only have one study to run, and who think that this one study might take a long time (certainly longer than a month).
For example, you might be running this study for a client and have little control over how much time it will take, or you just may be uncertain exactly when you need it for, and need the budget for the study signed off.
If you've run studies using our tools before, and are confident you can configure your study without needing to duplicate and iterate, then purchasing one study will suit you.
Why you might sign up for a monthly or yearly subscription
With subscription services you can run unlimited tests. What we see is that when people start seeing the results of their first study, they want to iterate and relaunch the original study to get more focused insights. Hence, people tend to prefer signing up for a subscription to one or more of our tools.
Subscribing to the Suite subscription also means you can run all four types of study (OptimalSort, Treejack, Questions and Chalkmark) on one design, and even chain studies together to make it easy for participants to complete your studies.