The verdict
If you sell time, or want to know which clients make money, pick Clockify. You can start on the free plan and build the habit before you pay for anything.
If your pain is things slipping through the cracks and you want one fast, shared to-do list, pick Todoist.
If you have both pains, run both. Todoist holds the tasks and Clockify holds the hours. They do not compete for the same job.
If I could only buy one for a small service business, I'd pick Clockify, because unbilled hours cost money and a task list can live in many places. If you work on fixed-price projects and never look at hours, Todoist is the better fit.