The verdict
Most readers will not need to choose between these two. Pick by what you sell. If you sell software, apps or other digital products to customers worldwide and want tax handled for you, I'd pick Paddle. If you sell billable work as an agency, consultancy or IT services firm and need to see profit per project, I'd pick Productive.
Some businesses do both, for example an agency with its own software product. In that case they sit in different parts of the stack. Productive covers the client work and its figures. Paddle covers the product's checkout and tax. Do not try to make one tool do both jobs.