Don't use company's that cache data and serve you old results from their other customers. For 2 reasons. 1) They are storing your data, this should be concerning. 2) You are paying for someone else's results. You paid for validation, you should get a validation in the moment, for you. That's how you get the best accuracy.
ZeroBounce does not serve you old results. They validate in real time, everytime, for every customer. I have tested it.
There can be a lot of factors to take into account, mainly you should aim to strike a balance between the quality of the service and prices. While looking for an email verification service best course of action is to test them with a list you know what the results should be, of varying types. I would recommend taking into account ZeroBounce as well, high accuracy rates of +98%, able to identify a wide range email statuses.