Because you're building a two-sided marketplace, your question would require validation of your hypothesis from your supply and demand base. Because there is a higher utility value and lower cost of customer acquisition with a subscription model, I would design around that payment model. So ask yourself (and your demand prospects) what subscription services would they value the most (create a list of top 5 to 10 services). Then, determine what just in time services would they value the most (again, creating a list of top 5 to 10). Build your supply based market around both subscription and just in time services. After you've done that, use an escrow payment model for JIT services.