Best billing model to choose for a project based product

Hi Product Hunt. We are trying to work out the best billing model for our platform (https://docbloc.co). It's a tool for API developers to manage and distribute API documentation. We have a limited free version at the moment and a Pro version on the way, but we need to decide on the billing model for the pro plans. Our 2 options are: Pro Account In this scenario the account creator will pay for a pro account and have access to pro features for all projects that they create, and plan will be based on usage etc. Because of the potential to have a lot of projects, the monthly/yearly rate would be quite high. Pro Project In this scenario the payments will be made per project, and only once a project has been upgraded will they be able to access premium features. The benefit of this is that there is no charge depending on the amount of users or other non project factors. The per project cost would be much lower than the option above as we know exactly what to expect. My instinct is to go for the Per Project billing model but would be great to hear some peoples thoughts on how they would tackle this.