The verdict
For most small B2B teams, I'd pick Clarity first. It costs no money and fixes problems you can see, such as a button that looks dead or a form that fails.
If you run a service business where leads call you, I'd pick CallRail. Your ad platform tells you who clicked, not who picked up the phone, and you may be cutting the keywords that make it ring.
If you have both a busy site and a busy phone line, use both. They do not overlap. Add Clarity today because the risk is nil, and add CallRail when you can show that calls matter.