From a business point of view they can be similar.
From the perspective of the mainframe guys, the whole point of a mainframe is that it is a single machine handling all of your transactions. Basically, it is simpler to deal with all kinds of transaction problems when you are not using a vastly distributed system with thousands of nodes. Typically PaaS/SaaS are large distributed systems.
To reliably and consistantly handle a very large stream of very important transactions where you basically need 100% reliability, they are a real option. The business case for a mainframe is something like, it would cost 200mln per year for some bank to make a failure proof distributed system, and 100mln to do it with a a mainframe. Outside of this type of systems, it is hard to think of any use for a mainframe, given the cost and complexity.