[grabeh]

With Simple, the banking services are provided by a third party bank, so Simple provides a usability layer on top of an existing bank's services.

With Mondo, they are looking to actually become a bank.

So on a usability level, you may not actually see much difference, but on another, it would seem like Mondo are fundamentally different as they will have a level of flexibility to decide which banking services to provide to customers. Although that's not so say that Simple couldn't use third party services, they may not have the same level of independence which Mondo wants/will have.