I like using the Product Matrix when comparing models (this one is February 2023):
Both options looks good based on your numbers. As typical ratio of traffic (which may differ in your case) is around 80% is SSL type. This may be a determining factor, if you want now or eventually to inspect this encrypted traffic with security policies, to select a more powerful model.
Correct sizing is also a prediction about the next few years as these devices will probably be there for a long time (5-7 years) and traffic always increase.
Start a discussion with your Fortinet reseller/Fortinet account team on the subject, they probably have good insight on the subject.