IaaS is the virtualization of computing, network, and storage running on premises or in the public cloud. A PaaS is an extra abstraction layer on top that allows you to work with services and applications. The abstraction layer of a PaaS will allow you to think in terms of services and applications while providing capabilities like auto-scaling, auto-healing, auto-provisioning, user governance, and optional high availability.
The choice of either an IaaS or a PaaS in relation to Mendix is based on the amount of applications you are planning to run and who the owner is of the PaaS layer on the cloud.
If you are able to consume a PaaS on demand from a public cloud or as a (virtual) private cloud, this will always be the best fit as a Mendix hosting solution based on cost, speed, and control.
In the scenario that you have access to an IaaS and you need to choose a PaaS layer yourself (like Kubernetes or Cloud Foundry), the key factor is the number of applications. A Kubernetes cluster can be beneficial when you are planning on running more than two applications, where each app contains a test, acceptance, and one or more production environments. When you are planning on running more than 10 applications, a Cloud Foundry solution could be beneficial (this has to do with the footprint and maintenance of the PaaS layer). Finally, for a single application, a traditional server-based solution will be enough.