Difference b/w FortiGate device with and without on board storage

Having an internal SSD in a xx1 model enables you to store logs locally, and not depend on a centralized FortiAnalyzer. This will enable the time period tabs on FortiView (which is confined to '5 min' without SSD).

I am not sure if it enables WAN caching at the same time but definitively as an alternative.

I personally would only consider a xx1 model below a FG-100E. The model itself is more expensive, and each and every support contract during it's lifetime. A FAZ has a lot more options, reports, ability to create reports etc. and might cost the same over a period of some years.