I haven't spent much time looking at PayPal's API, but it looks like that more of a competitor to something like braintree or authorize.net. It's just for processing the payment, right? The services I'm looking at here manage the entire subscription process. Essentially, you're outsourcing your billing system to them so that you don't have to think about things like dealing with the dunning process or prorating the bill if the user upgrades their plan mid-month, etc.