It’s really going to come down to what you’re comfortable using and what you’re comfortable paying.
If cPanel is working for you and you are ok with the cost, then moving to another panel is a preference that is completely up to you.
Personally, just to host WHMCS, I would also consider security a priority as well, which most of the free panels available seriously lack, so choosing a free panel wouldn’t be an option.
If I had to make a choice on a panel, I would go with either Webuzo or DirectAdmin.