The verdict
Most readers will not choose between these. If your trouble is that clients pay late or you have no easy way to take a payment, I'd start with PayPal as a quick way to send links and invoices. If your trouble is that you cannot tell which projects and clients make money, or who has capacity, I'd pick Productive.
An agency can run both. Productive covers the work and the figures. PayPal can be one way a client pays. Do not expect PayPal to forecast your revenue, or Productive to act as a payment provider. Check the fee table for your country before you rely on PayPal, and run Productive's trial on a real project.