Rocket Money is a full personal finance app. PayPing is laser-focused on subscription tracking with better pricing.
Rocket Money tries to be everything: budgeting, credit monitoring, bill negotiation, and subscription tracking. PayPing does one thing exceptionally well: track your subscriptions. Less complexity means fewer features to learn and a cleaner interface.
Rocket Money's premium features cost $3-12/month ($36-144/year). PayPing's lifetime deal is $29 total. If you use it for more than 2-8 months, PayPing is cheaper. Forever.