With the Software-as-a-Service (SaaS) model, you are renting the software and hardware services based on the number of users that access the system. The monthly rental cost includes all the software license fees, software updates and the rental of the server space and time. You are however responsible for the PCs, peripherals, and connectivity required to access the software application from your premises.
A perpetual licence is the ‘traditional’ model used to purchase software. You pay for your software licence up-front and have the right to use it indefinitely. On top of the licence fee, you will have the option to pay for one-off implementation services and a support contract, which is renewed annually.